Duodecimal

The duodecimal system (also known as base 12, dozenal, or, rarely, uncial) is a positional notation numeral system using twelve as its base. The number twelve (that is, the number written as "12" in the base ten numerical system) is instead written as "10" in duodecimal (meaning "1 dozen and 0 units", instead of "1 ten and 0 units"), whereas the digit string "12" means "1 dozen and 2 units" (i.e. the same number that in decimal is written as "14"). Similarly, in duodecimal "100" means "1 gross", "1000" means "1 great gross", and "0.1" means "1 twelfth" (instead of their decimal meanings "1 hundred", "1 thousand", and "1 tenth").

Various symbols have been used to stand for ten and eleven in duodecimal notation; this page uses A and B, as in hexadecimal, which make a duodecimal count from zero to twelve read 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, 10.

The number twelve, a superior highly composite number, is the smallest number with four non-trivial factors (2, 3, 4, 6), and the smallest to include as factors all four numbers (1 to 4) within the subitizing range, and the smallest abundant number. All multiples of reciprocals of 3-smooth numbers ( $.mw-parser-output .frac{white-space:nowrap}.mw-parser-output .frac .num,.mw-parser-output .frac .den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output .frac .den{vertical-align:sub}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}a⁄2b·3c$ where $a,b,c$ are integers) have a terminating representation in duodecimal. In particular, +1⁄4 (0.3), +1⁄3 (0.4), +1⁄2 (0.6), +2⁄3 (0.8), and +3⁄4 (0.9) all have a short terminating representation in duodecimal. There is also higher regularity observable in the duodecimal multiplication table. As a result, duodecimal has been described as the optimal number system.^[1]

This is considered superior to base-10 (which has only 2 and 5 as factors), and also to other proposed bases such as 16 or 20. Base-60 (and the less popular base-30) do even better in this respect (the reciprocals of all 5-smooth numbers terminate) but at the cost of unwieldy multiplication tables and a much larger number of symbols to memorize.

Origin[]

In this section, numerals are based on decimal places. For example, 10 means ten, 12 means twelve.

Languages using duodecimal number systems are uncommon. Languages in the Nigerian Middle Belt such as Janji, Gbiri-Niragu (Gure-Kahugu), Piti, and the Nimbia dialect of Gwandara;^[2] and the Chepang language of Nepal^[3] are known to use duodecimal numerals.

Germanic languages have special words for 11 and 12, such as eleven and twelve in English. They come from Proto-Germanic *ainlif and *twalif (meaning, respectively one left and two left), suggesting a decimal rather than duodecimal origin.^[4]^[5] However, Old Norse used a duodecimal counting system, with its words for "one hundred and eighty" meaning 200 and "two hundred" meaning 240.^[6] On British Isles, this style of counting survived well into the middle ages as the long hundred.

Historically, units of time in many civilizations are duodecimal. There are twelve signs of the zodiac, twelve months in a year, and the Babylonians had twelve hours in a day (although at some point this was changed to 24). Traditional Chinese calendars, clocks, and compasses are based on the twelve Earthly Branches. There are 12 inches in an imperial foot, 12 troy ounces in a troy pound, 12 old British pence in a shilling, 24 (12×2) hours in a day, and many other items counted by the dozen, gross (144, square of 12), or great gross (1728, cube of 12). The Romans used a fraction system based on 12, including the uncia which became both the English words ounce and inch. Pre-decimalisation, Ireland and the United Kingdom used a mixed duodecimal-vigesimal currency system (12 pence = 1 shilling, 20 shillings or 240 pence to the pound sterling or Irish pound), and Charlemagne established a monetary system that also had a mixed base of twelve and twenty, the remnants of which persist in many places.

Table of units from a base of 12
Relative value	French unit of length	English unit of length	English (Troy) unit of weight	Roman unit of weight	English unit of mass
12⁰	pied	foot	pound	libra
12⁻¹	pouce	inch	ounce	uncia	slinch
12⁻²	ligne	line	2 scruples	2 scrupula	slug
12⁻³	point	point	seed	siliqua

The importance of 12 has been attributed to the number of lunar cycles in a year as well as the fact that humans have 12 finger bones (phalanges) on one hand (three in each of four fingers).^[7]^[8] It is possible to count to 12 with the thumb acting as a pointer, touching each finger bone in turn. A traditional finger counting system still in use in many regions of Asia works in this way and could help to explain the occurrence of numeral systems based on 12 and 60 besides those based on 10, 20, and 5. In this system, the one (usually right) hand counts repeatedly to 12, displaying the number of iterations on the other (usually left), until five dozens, i.e. the 60, are full.^[9]^[10]

Notations and pronunciations[]

Transdecimal symbols[]

In a numbering system the base (twelve for duodecimal) must be written as 10, but there are numerous proposals for how to write duodecimal ten and eleven.^[11]

To allow entry on typewriters, letters such as A and B (as in hexadecimal), T and E (initials of Ten and Eleven), X and E (X from the Roman numeral for ten), or X and Z are used. Some employ Greek letters such as δ (standing for Greek δέκα 'ten') and ε (for Greek ένδεκα 'eleven'), or τ and ε.^[11] Frank Emerson Andrews, an early American advocate for duodecimal, suggested and used in his book New Numbers an X and ℰ (script E, U+2130).^[12]

Edna Kramer in her 1951 book The Main Stream of Mathematics used a six-pointed asterisk (sextile) ⚹ and a hash (or octothorpe) #.^[11] The symbols were chosen because they were available on some typewriters; they are also on push-button telephones.^[11] This notation was used in publications of the Dozenal Society of America (DSA) from 1974–2008.^[13]^[14]

From 2008 to 2015, the DSA used and , the symbols devised by William Addison Dwiggins.^[11]^[15]

The Dozenal Society of Great Britain (DSGB) proposed symbols and .^[11] This notation, derived from Arabic digits by 180° rotation, was introduced by Isaac Pitman.^[16]^[11]^[17] In March 2013, a proposal was submitted to include the digit forms for ten and eleven propagated by the Dozenal Societies in the Unicode Standard.^[18] Of these, the British/Pitman forms were accepted for encoding as characters at code points U+218A ↊ TURNED DIGIT TWO and U+218B ↋ TURNED DIGIT THREE. They were included in the Unicode 8.0 release in June 2015^[19]^[20] and are available in LaTeX as \textturntwo and \textturnthree.^[21]

After the Pitman digits were added to Unicode, the DSA took a vote and then began publishing content using the Pitman digits instead.^[22] They still use the letters X and E in ASCII text. As the Unicode characters are poorly supported, this page uses "A" and "B".

Other proposals are more creative or aesthetic; for example, many do not use any Arabic numerals under the principle of "separate identity."^[11]

Base notation[]

There are also varying proposals of how to distinguish a duodecimal number from a decimal one.^[23] They include italicizing duodecimal numbers "54 = 64", adding a "Humphrey point" (a semicolon instead of a decimal point) to duodecimal numbers "54;6 = 64.5", or some combination of the two. Others use subscript or affixed labels to indicate the base, allowing for more than decimal and duodecimal to be represented (for single letters 'z' from "dozenal" is used as 'd' would mean decimal)^[23] such as "54_z = 64_d," "54₁₂ = 64₁₀" or "doz 54 = dec 64."

Pronunciation[]

The Dozenal Society of America suggested the pronunciation of ten and eleven as "dek" and "el". For the names of powers of twelve there are two prominent systems.

Do-gro-mo system[]

In this system, the prefix e- is added for fractions.^[15]^[24]

Duodecimal	Name	Decimal	Duodecimal fraction	Name
1;	one	1
10;	do	12	0;1	edo
100;	gro	144	0;01	egro
1,000;	mo	1,728	0;001	emo
10,000;	do-mo	20,736	0;000,1	edo-mo
100,000;	gro-mo	248,832	0;000,01	egro-mo
1,000,000;	bi-mo	2,985,984	0;000,001	ebi-mo
10,000,000;	do-bi-mo	35,831,808	0;000,000,1	edo-bi-mo
100,000,000;	gro-bi-mo	429,981,696	0;000,000,01	egro-bi-mo
1,000,000,000;	tri-mo	5,159,780,352	0;000,000,001	etri-mo
10,000,000,000;	do-tri-mo	61,917,364,224	0;000,000,000,1	edo-tri-mo
100,000,000,000;	gro-tri-mo	743,008,370,688	0;000,000,000,01	egro-tri-mo
1,000,000,000,000;	quad-mo	8,916,100,448,256	0;000,000,000,001	equad-mo
10,000,000,000,000;	do-quad-mo	106,993,205,379,072	0;000,000,000,000,1	edo-quad-mo
100,000,000,000,000;	gro-quad-mo	1,283,918,464,548,864	0;000,000,000,000,01	egro-quad-mo
1,000,000,000,000,000;	penta-mo	15,407,021,574,586,368	0;000,000,000,000,001	epenta-mo
10,000,000,000,000,000;	do-penta-mo	184,884,258,895,036,416	0;000,000,000,000,000,1	edo-penta-mo
100,000,000,000,000,000;	gro-penta-mo	2,218,611,106,740,436,992	0;000,000,000,000,000,01	egro-penta-mo
1,000,000,000,000,000,000;	hexa-mo	26,623,333,280,885,243,904	0;000,000,000,000,000,001	ehexa-mo

Multiple digits in this series are pronounced differently: 12 is "do two"; 30 is "three do"; 100 is "gro"; BA9 is "el gro dek do nine"; B86 is "el gro eight do six"; 8BB,15A is "eight gro el do el, one gro five do dek"; and so on.^[24]

Systematic Dozenal Nomenclature (SDN)[]

This system uses "-qua" ending for the positive powers of 12 and "-cia" ending for the negative powers of 12, and an extension of the IUPAC systematic element names (with syllables dec and lev for the two extra digits needed for duodecimal) to express which power is meant.^[25]^[26]

Duodecimal	Name	Decimal	Duodecimal fraction	Name
1;	one	1
10;	unqua	12	0;1	uncia
100;	biqua	144	0;01	bicia
1,000;	triqua	1,728	0;001	tricia
10,000;	quadqua	20,736	0;000,1	quadcia
100,000;	pentqua	248,832	0;000,01	pentcia
1,000,000;	hexqua	2,985,984	0;000,001	hexcia
10,000,000;	septqua	35,831,808	0;000,000,1	septcia
100,000,000;	octqua	429,981,696	0;000,000,01	octcia
1,000,000,000;	ennqua	5,159,780,352	0;000,000,001	enncia
10,000,000,000;	decqua	61,917,364,224	0;000,000,000,1	deccia
100,000,000,000;	levqua	743,008,370,688	0;000,000,000,01	levcia
1,000,000,000,000;	unnilqua	8,916,100,448,256	0;000,000,000,001	unnilcia
10,000,000,000,000;	ununqua	106,993,205,379,072	0;000,000,000,000,1	ununcia

Advocacy and "dozenalism"[]

William James Sidis used 12 as the base for his constructed language Vendergood in 1906, noting it being the smallest number with four factors and its prevalence in commerce.^[27]

The case for the duodecimal system was put forth at length in Frank Emerson Andrews' 1935 book New Numbers: How Acceptance of a Duodecimal Base Would Simplify Mathematics. Emerson noted that, due to the prevalence of factors of twelve in many traditional units of weight and measure, many of the computational advantages claimed for the metric system could be realized either by the adoption of ten-based weights and measure or by the adoption of the duodecimal number system.^[12]

A duodecimal clockface as in the logo of the Dozenal Society of America, here used to denote musical keys

Both the Dozenal Society of America and the Dozenal Society of Great Britain promote widespread adoption of the base-twelve system. They use the word "dozenal" instead of "duodecimal" to avoid the more overtly base-ten terminology. However, the etymology of "dozenal" itself is also an expression based on base-ten terminology since "dozen" is a direct derivation of the French word douzaine which is a derivative of the French word for twelve, douze, descended from Latin duodecim.

Since at least as far back as 1945 some members of the Dozenal Society of America and Dozenal Society of Great Britain have suggested that a more apt word would be "uncial". Uncial is a derivation of the Latin word uncia, meaning "one-twelfth", and also the base-twelve analogue of the Latin word decima, meaning "one-tenth".^[28]

Mathematician and mental calculator Alexander Craig Aitken was an outspoken advocate of duodecimal:

The duodecimal tables are easy to master, easier than the decimal ones; and in elementary teaching they would be so much more interesting, since young children would find more fascinating things to do with twelve rods or blocks than with ten. Anyone having these tables at command will do these calculations more than one-and-a-half times as fast in the duodecimal scale as in the decimal. This is my experience; I am certain that even more so it would be the experience of others.
— A. C. Aitken, "Twelves and Tens" in The Listener (January 25, 1962)^[29]

But the final quantitative advantage, in my own experience, is this: in varied and extensive calculations of an ordinary and not unduly complicated kind, carried out over many years, I come to the conclusion that the efficiency of the decimal system might be rated at about 65 or less, if we assign 100 to the duodecimal.
— A. C. Aitken, The Case Against Decimalisation (1962)^[30]

In media[]

In "Little Twelvetoes", American television series Schoolhouse Rock! portrayed an alien being using base-twelve arithmetic, using "dek", "el" and "doh" as names for ten, eleven and twelve, and Andrews' script-X and script-E for the digit symbols.^[31]^[32]

Duodecimal systems of measurements[]

Systems of measurement proposed by dozenalists include:

Tom Pendlebury's TGM system^[33]^[26]
Takashi Suga's Universal Unit System^[34]^[26]
John Volan's Primel system^[35]

Comparison to other number systems[]

The number 12 has six factors, which are 1, 2, 3, 4, 6, and 12, of which 2 and 3 are prime. The decimal system has only four factors, which are 1, 2, 5, and 10, of which 2 and 5 are prime.

Vigesimal (base 20) adds two factors to those of ten, namely 4 and 20, but no additional prime factor. Octodecimal (base 18) adds two factors to those of six, namely 9 and 18. Although twenty and eighteen has 6 factors, 2 of them prime, similarly to twelve, it is also a much larger base, and so the digit set and the multiplication table are much larger.

Binary has only two factors, 1 and 2, the latter being prime. Hexadecimal (base 16) has five factors, adding 4, 8 and 16 to those of 2, but no additional prime. Sixteen has no odd factors, thus divisible only by power of 2, and it is also a much larger base, and so the digit set and the multiplication table are much larger.

Trigesimal (base 30) is the smallest system that has three different prime factors (all of the three smallest primes: 2, 3 and 5) and it has eight factors in total (1, 2, 3, 5, 6, 10, 15, and 30). Sexagesimal — which the ancient Sumerians and Babylonians among others actually used—adds the four convenient factors 4, 12, 20, and 60 to this but no new prime factors. The smallest system that has four different prime factors is base 210 and the pattern follows the primorials. In all base systems, there are similarities to the representation of multiples of numbers which are one less than or one more than the base.

Duodecimal multiplication table
×	1	2	3	4	5	6	7	8	9	A	B	10
1	1	2	3	4	5	6	7	8	9	A	B	10
2	2	4	6	8	A	10	12	14	16	18	1A	20
3	3	6	9	10	13	16	19	20	23	26	29	30
4	4	8	10	14	18	20	24	28	30	34	38	40
5	5	A	13	18	21	26	2B	34	39	42	47	50
6	6	10	16	20	26	30	36	40	46	50	56	60
7	7	12	19	24	2B	36	41	48	53	5A	65	70
8	8	14	20	28	34	40	48	54	60	68	74	80
9	9	16	23	30	39	46	53	60	69	76	83	90
A	A	18	26	34	42	50	5A	68	76	84	92	A0
B	B	1A	29	38	47	56	65	74	83	92	A1	B0
10	10	20	30	40	50	60	70	80	90	A0	B0	100

Conversion tables to and from decimal[]

To convert numbers between bases, one can use the general conversion algorithm (see the relevant section under positional notation). Alternatively, one can use digit-conversion tables. The ones provided below can be used to convert any duodecimal number between 0;01 and BBB,BBB;BB to decimal, or any decimal number between 0.01 and 999,999.99 to duodecimal. To use them, the given number must first be decomposed into a sum of numbers with only one significant digit each. For example:

123,456.78 = 100,000 + 20,000 + 3,000 + 400 + 50 + 6 + 0.7 + 0.08

This decomposition works the same no matter what base the number is expressed in. Just isolate each non-zero digit, padding them with as many zeros as necessary to preserve their respective place values. If the digits in the given number include zeroes (for example, 102,304.05), these are, of course, left out in the digit decomposition (102,304.05 = 100,000 + 2,000 + 300 + 4 + 0.05). Then the digit conversion tables can be used to obtain the equivalent value in the target base for each digit. If the given number is in duodecimal and the target base is decimal, we get:

(duodecimal) 100,000 + 20,000 + 3,000 + 400 + 50 + 6 + 0;7 + 0;08 = (decimal) 248,832 + 41,472 + 5,184 + 576 + 60 + 6 + 0.583333333333... + 0.055555555555...

Now, because the summands are already converted to base ten, the usual decimal arithmetic is used to perform the addition and recompose the number, arriving at the conversion result:

Duodecimal  ----->  Decimal

  100,000     =    248,832
   20,000     =     41,472
    3,000     =      5,184
      400     =        576
       50     =         60
 +      6     =   +      6
        0;7   =          0.583333333333...
        0;08  =          0.055555555555...
--------------------------------------------
  123,456;78  =    296,130.638888888888...

That is, (duodecimal) 123,456.78 equals (decimal) 296,130.638 ≈ 296,130.64

If the given number is in decimal and the target base is duodecimal, the method is basically same. Using the digit conversion tables:

(decimal) 100,000 + 20,000 + 3,000 + 400 + 50 + 6 + 0.7 + 0.08 = (duodecimal) 49,A54 + B,6A8 + 1,8A0 + 294 + 42 + 6 + 0;849724972497249724972497... + 0;0B62A68781B05915343A0B62...

However, in order to do this sum and recompose the number, now the addition tables for the duodecimal system have to be used, instead of the addition tables for decimal most people are already familiar with, because the summands are now in base twelve and so the arithmetic with them has to be in duodecimal as well. In decimal, 6 + 6 equals 12, but in duodecimal it equals 10; so, if using decimal arithmetic with duodecimal numbers one would arrive at an incorrect result. Doing the arithmetic properly in duodecimal, one gets the result:

  Decimal  ----->  Duodecimal

  100,000     =     49,A54
   20,000     =      B,6A8
    3,000     =      1,8A0
      400     =        294
       50     =         42
 +      6     =   +      6
        0.7   =          0;849724972497249724972497...
        0.08  =          0;0B62A68781B05915343A0B62...
--------------------------------------------------------
  123,456.78  =     5B,540;943A0B62A68781B05915343A...

That is, (decimal) 123,456.78 equals (duodecimal) 5B,540;943A0B62A68781B059153... ≈ 5B,540;94

Duodecimal to decimal digit conversion[]

Duod.	Decimal	Duod.	Decimal	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.
1,000,000	2,985,984	100,000	248,832	10,000	20,736	1,000	1,728	100	144	10	12	1	1	0;1	0.083	0;01	0.00694
2,000,000	5,971,968	200,000	497,664	20,000	41,472	2,000	3,456	200	288	20	24	2	2	0;2	0.16	0;02	0.0138
3,000,000	8,957,952	300,000	746,496	30,000	62,208	3,000	5,184	300	432	30	36	3	3	0;3	0.25	0;03	0.02083
4,000,000	11,943,936	400,000	995,328	40,000	82,944	4,000	6,912	400	576	40	48	4	4	0;4	0.3	0;04	0.027
5,000,000	14,929,920	500,000	1,244,160	50,000	103,680	5,000	8,640	500	720	50	60	5	5	0;5	0.416	0;05	0.03472
6,000,000	17,915,904	600,000	1,492,992	60,000	124,416	6,000	10,368	600	864	60	72	6	6	0;6	0.5	0;06	0.0416
7,000,000	20,901,888	700,000	1,741,824	70,000	145,152	7,000	12,096	700	1,008	70	84	7	7	0;7	0.583	0;07	0.04861
8,000,000	23,887,872	800,000	1,990,656	80,000	165,888	8,000	13,824	800	1,152	80	96	8	8	0;8	0.6	0;08	0.05
9,000,000	26,873,856	900,000	2,239,488	90,000	186,624	9,000	15,552	900	1,296	90	108	9	9	0;9	0.75	0;09	0.0625
A,000,000	29,859,840	A00,000	2,488,320	A0,000	207,360	A,000	17,280	A00	1,440	A0	120	A	10	0;A	0.83	0;0A	0.0694
B,000,000	32,845,824	B00,000	2,737,152	B0,000	228,096	B,000	19,008	B00	1,584	B0	132	B	11	0;B	0.916	0;0B	0.07638

Decimal to duodecimal digit conversion[]

Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duod.	Dec.	Duodecimal	Dec.	Duodecimal
1,000,000	402,854	100,000	49,A54	10,000	5,954	1,000	6B4	100	84	10	A	1	1	0.1	0;12497	0.01	0;015343A0B62A68781B059
2,000,000	805,4A8	200,000	97,8A8	20,000	B,6A8	2,000	1,1A8	200	148	20	18	2	2	0.2	0;2497	0.02	0;02A68781B05915343A0B6
3,000,000	1,008,140	300,000	125,740	30,000	15,440	3,000	1,8A0	300	210	30	26	3	3	0.3	0;37249	0.03	0;043A0B62A68781B059153
4,000,000	1,40A,994	400,000	173,594	40,000	1B,194	4,000	2,394	400	294	40	34	4	4	0.4	0;4972	0.04	0;05915343A0B62A68781B
5,000,000	1,811,628	500,000	201,428	50,000	24,B28	5,000	2,A88	500	358	50	42	5	5	0.5	0;6	0.05	0;07249
6,000,000	2,014,280	600,000	24B,280	60,000	2A,880	6,000	3,580	600	420	60	50	6	6	0.6	0;7249	0.06	0;08781B05915343A0B62A6
7,000,000	2,416,B14	700,000	299,114	70,000	34,614	7,000	4,074	700	4A4	70	5A	7	7	0.7	0;84972	0.07	0;0A0B62A68781B05915343
8,000,000	2,819,768	800,000	326,B68	80,000	3A,368	8,000	4,768	800	568	80	68	8	8	0.8	0;9724	0.08	0;0B62A68781B05915343A
9,000,000	3,020,400	900,000	374,A00	90,000	44,100	9,000	5,260	900	630	90	76	9	9	0.9	0;A9724	0.09	0;10B62A68781B05915343A

Divisibility rules[]

(In this section, all numbers are written with duodecimal)

This section is about the divisibility rules in duodecimal.

1

Any integer is divisible by 1.

2

If a number is divisible by 2 then the unit digit of that number will be 0, 2, 4, 6, 8 or A.

3

If a number is divisible by 3 then the unit digit of that number will be 0, 3, 6 or 9.

4

If a number is divisible by 4 then the unit digit of that number will be 0, 4 or 8.

5

To test for divisibility by 5, double the units digit and subtract the result from the number formed by the rest of the digits. If the result is divisible by 5 then the given number is divisible by 5.

This rule comes from 21(5*5)

Examples:
13 rule => |1-2*3| = 5 which is divisible by 5.
2BA5 rule => |2BA-2*5| = 2B0(5*70) which is divisible by 5(or apply the rule on 2B0).

OR

To test for divisibility by 5, subtract the units digit and triple of the result to the number formed by the rest of the digits. If the result is divisible by 5 then the given number is divisible by 5.

This rule comes from 13(5*3)

Examples:
13 rule => |3-3*1| = 0 which is divisible by 5.
2BA5 rule => |5-3*2BA| = 8B1(5*195) which is divisible by 5(or apply the rule on 8B1).

OR

Form the alternating sum of blocks of two from right to left. If the result is divisible by 5 then the given number is divisible by 5.

This rule comes from 101, since 101 = 5*25, thus this rule can be also tested for the divisibility by 25.

Example:

97,374,627 => 27-46+37-97 = -7B which is divisible by 5.

6

If a number is divisible by 6 then the unit digit of that number will be 0 or 6.

7

To test for divisibility by 7, triple the units digit and add the result to the number formed by the rest of the digits. If the result is divisible by 7 then the given number is divisible by 7.

This rule comes from 2B(7*5)

Examples:
12 rule => |3*2+1| = 7 which is divisible by 7.
271B rule => |3*B+271| = 29A(7*4A) which is divisible by 7(or apply the rule on 29A).

OR

To test for divisibility by 7, subtract the units digit and double the result from the number formed by the rest of the digits. If the result is divisible by 7 then the given number is divisible by 7.

This rule comes from 12(7*2)

Examples:
12 rule => |2-2*1| = 0 which is divisible by 7.
271B rule => |B-2*271| = 513(7*89) which is divisible by 7(or apply the rule on 513).

OR

To test for divisibility by 7, 4 times the units digit and subtract the result from the number formed by the rest of the digits. If the result is divisible by 7 then the given number is divisible by 7.

This rule comes from 41(7*7)

Examples:
12 rule => |4*2-1| = 7 which is divisible by 7.
271B rule => |4*B-271| = 235(7*3B) which is divisible by 7(or apply the rule on 235).

OR

Form the alternating sum of blocks of three from right to left. If the result is divisible by 7 then the given number is divisible by 7.

This rule comes from 1001, since 1001 = 7*11*17, thus this rule can be also tested for the divisibility by 11 and 17.

Example:

386,967,443 => 443-967+386 = -168 which is divisible by 7.

8

If the 2-digit number formed by the last 2 digits of the given number is divisible by 8 then the given number is divisible by 8.

Example: 1B48, 4120

     rule => since 48(8*7) divisible by 8, then 1B48 is divisible by 8.
     rule => since 20(8*3) divisible by 8, then 4120 is divisible by 8.

9

If the 2-digit number formed by the last 2 digits of the given number is divisible by 9 then the given number is divisible by 9.

Example: 7423, 8330

     rule => since 23(9*3) divisible by 9, then 7423 is divisible by 9.
     rule => since 30(9*4) divisible by 9, then 8330 is divisible by 9.

A

If the number is divisible by 2 and 5 then the number is divisible by A.

B

If the sum of the digits of a number is divisible by B then the number is divisible by B (the equivalent of casting out nines in decimal).

Example: 29, 61B13

     rule => 2+9 = B which is divisible by B, then 29 is divisible by B.
     rule => 6+1+B+1+3 = 1A which is divisible by B, then 61B13 is divisible by B.

10

If a number is divisible by 10 then the unit digit of that number will be 0.

11

Sum the alternate digits and subtract the sums. If the result is divisible by 11 the number is divisible by 11 (the equivalent of divisibility by eleven in decimal).

Example: 66, 9427

     rule => |6-6| = 0 which is divisible by 11, then 66 is divisible by 11.
     rule => |(9+2)-(4+7)| = |A-A| = 0 which is divisible by 11, then 9427 is divisible by 11.

12

If the number is divisible by 2 and 7 then the number is divisible by 12.

13

If the number is divisible by 3 and 5 then the number is divisible by 13.

14

If the 2-digit number formed by the last 2 digits of the given number is divisible by 14 then the given number is divisible by 14.

Example: 1468, 7394

     rule => since 68(14*5) divisible by 14, then 1468 is divisible by 14.
     rule => since 94(14*7) divisible by 14, then 7394 is divisible by 14.

Fractions and irrational numbers[]

Fractions[]

Duodecimal fractions may be simple:

1/2 = 0;6
1/3 = 0;4
1/4 = 0;3
1/6 = 0;2
1/8 = 0;16
1/9 = 0;14
1/10 = 0;1 (this is a twelfth, 1/A is a tenth)
1/14 = 0;09 (this is a sixteenth, 1/12 is a fourteenth)

or complicated:

1/5 = 0;249724972497... recurring (rounded to 0.24A)
1/7 = 0;186A35186A35... recurring (rounded to 0.187)
1/A = 0;1249724972497... recurring (rounded to 0.125)
1/B = 0;111111111111... recurring (rounded to 0.111)
1/11 = 0;0B0B0B0B0B0B... recurring (rounded to 0.0B1)
1/12 = 0;0A35186A35186... recurring (rounded to 0.0A3)
1/13 = 0;0972497249724... recurring (rounded to 0.097)

Examples in duodecimal	Decimal equivalent
1 × (5/8) = 0.76	1 × (5/8) = 0.625
100 × (5/8) = 76	144 × (5/8) = 90
576/9 = 76	810/9 = 90
400/9 = 54	576/9 = 64
1A.6 + 7.6 = 26	22.5 + 7.5 = 30

As explained in recurring decimals, whenever an irreducible fraction is written in radix point notation in any base, the fraction can be expressed exactly (terminates) if and only if all the prime factors of its denominator are also prime factors of the base.

Thus, in decimal (= 2 × 5) system, fractions whose denominators are made up solely of multiples of 2 and 5 terminate: 1/8 = 1/(2×2×2), 1/20 = 1/(2×2×5) and 1/500 = 1/(2×2×5×5×5) can be expressed exactly as 0.125, 0.05 and 0.002 respectively. 1/3 and 1/7, however, recur (0.333... and 0.142857142857...).

In the duodecimal (= 2 × 2 × 3) system, 1/8 is exact; 1/20 and 1/500 recur because they include 5 as a factor; 1/3 is exact; and 1/7 recurs, just as it does in decimal.

The number of denominators which give terminating fractions within a given number of digits, say n, in a base b is the number of factors (divisors) of bⁿ, the nth power of the base b (although this includes the divisor 1, which does not produce fractions when used as the denominator). The number of factors of bⁿ is given using its prime factorization.

For decimal, 10ⁿ = 2ⁿ × 5ⁿ. The number of divisors is found by adding one to each exponent of each prime and multiplying the resulting quantities together, so the number of factors of 10ⁿ is (n + 1)(n + 1) = (n + 1)².

For example, the number 8 is a factor of 10³ (1000), so 1/8 and other fractions with a denominator of 8 cannot require more than 3 fractional decimal digits to terminate. 5/8 = 0.625₁₀

For duodecimal, 10ⁿ = 2²ⁿ × 3ⁿ. This has (2n + 1)(n + 1) divisors. The sample denominator of 8 is a factor of a gross (12² = 144 in decimal), so eighths cannot need more than two duodecimal fractional places to terminate. 5/8 = 0.76₁₂

Because both ten and twelve have two unique prime factors, the number of divisors of bⁿ for b = 10 or 12 grows quadratically with the exponent n (in other words, of the order of n²).

Recurring digits[]

The Dozenal Society of America argues that factors of 3 are more commonly encountered in real-life division problems than factors of 5.^[36] Thus, in practical applications, the nuisance of repeating decimals is encountered less often when duodecimal notation is used. Advocates of duodecimal systems argue that this is particularly true of financial calculations, in which the twelve months of the year often enter into calculations.

However, when recurring fractions do occur in duodecimal notation, they are less likely to have a very short period than in decimal notation, because 12 (twelve) is between two prime numbers, 11 (eleven) and 13 (thirteen), whereas ten is adjacent to the composite number 9. Nonetheless, having a shorter or longer period doesn't help the main inconvenience that one does not get a finite representation for such fractions in the given base (so rounding, which introduces inexactitude, is necessary to handle them in calculations), and overall one is more likely to have to deal with infinite recurring digits when fractions are expressed in decimal than in duodecimal, because one out of every three consecutive numbers contains the prime factor 3 in its factorization, whereas only one out of every five contains the prime factor 5. All other prime factors, except 2, are not shared by either ten or twelve, so they do not influence the relative likeliness of encountering recurring digits (any irreducible fraction that contains any of these other factors in its denominator will recur in either base).

Also, the prime factor 2 appears twice in the factorization of twelve, whereas only once in the factorization of ten; which means that most fractions whose denominators are powers of two will have a shorter, more convenient terminating representation in duodecimal than in decimal:

1/(2²) = 0.25₁₀ = 0.3₁₂
1/(2³) = 0.125₁₀ = 0.16₁₂
1/(2⁴) = 0.0625₁₀ = 0.09₁₂
1/(2⁵) = 0.03125₁₀ = 0.046₁₂

Fraction	Prime factors of the denominator	Positional representation	Positional representation	Prime factors of the denominator	Fraction
Decimal base Prime factors of the base: 2, 5 Prime factors of one below the base: 3 Prime factors of one above the base: 11 All other primes: 7, 13, 17, 19, 23, 29, 31			Duodecimal base Prime factors of the base: 2, 3 Prime factors of one below the base: B Prime factors of one above the base: 11 All other primes: 5, 7, 15, 17, 1B, 25, 27
1/2	2	0.5	0;6	2	1/2
1/3	3	0.3	0;4	3	1/3
1/4	2	0.25	0;3	2	1/4
1/5	5	0.2	0;2497	5	1/5
1/6	2, 3	0.16	0;2	2, 3	1/6
1/7	7	0.142857	0;186A35	7	1/7
1/8	2	0.125	0;16	2	1/8
1/9	3	0.1	0;14	3	1/9
1/10	2, 5	0.1	0;12497	2, 5	1/A
1/11	11	0.09	0;1	B	1/B
1/12	2, 3	0.083	0;1	2, 3	1/10
1/13	13	0.076923	0;0B	11	1/11
1/14	2, 7	0.0714285	0;0A35186	2, 7	1/12
1/15	3, 5	0.06	0;09724	3, 5	1/13
1/16	2	0.0625	0;09	2	1/14
1/17	17	0.0588235294117647	0;08579214B36429A7	15	1/15
1/18	2, 3	0.05	0;08	2, 3	1/16
1/19	19	0.052631578947368421	0;076B45	17	1/17
1/20	2, 5	0.05	0;07249	2, 5	1/18
1/21	3, 7	0.047619	0;06A3518	3, 7	1/19
1/22	2, 11	0.045	0;06	2, B	1/1A
1/23	23	0.0434782608695652173913	0;06316948421	1B	1/1B
1/24	2, 3	0.0416	0;06	2, 3	1/20
1/25	5	0.04	0;05915343A0B62A68781B	5	1/21
1/26	2, 13	0.0384615	0;056	2, 11	1/22
1/27	3	0.037	0;054	3	1/23
1/28	2, 7	0.03571428	0;05186A3	2, 7	1/24
1/29	29	0.0344827586206896551724137931	0;04B7	25	1/25
1/30	2, 3, 5	0.03	0;04972	2, 3, 5	1/26
1/31	31	0.032258064516129	0;0478AA093598166B74311B28623A55	27	1/27
1/32	2	0.03125	0;046	2	1/28
1/33	3, 11	0.03	0;04	3, B	1/29
1/34	2, 17	0.02941176470588235	0;0429A708579214B36	2, 15	1/2A
1/35	5, 7	0.0285714	0;0414559B3931	5, 7	1/2B
1/36	2, 3	0.027	0;04	2, 3	1/30

The duodecimal period length of 1/n are (in base 10)

0, 0, 0, 0, 4, 0, 6, 0, 0, 4, 1, 0, 2, 6, 4, 0, 16, 0, 6, 4, 6, 1, 11, 0, 20, 2, 0, 6, 4, 4, 30, 0, 1, 16, 12, 0, 9, 6, 2, 4, 40, 6, 42, 1, 4, 11, 23, 0, 42, 20, 16, 2, 52, 0, 4, 6, 6, 4, 29, 4, 15, 30, 6, 0, 4, 1, 66, 16, 11, 12, 35, 0, ... (sequence A246004 in the OEIS)

The duodecimal period length of 1/(nth prime) are (in base 10)

0, 0, 4, 6, 1, 2, 16, 6, 11, 4, 30, 9, 40, 42, 23, 52, 29, 15, 66, 35, 36, 26, 41, 8, 16, 100, 102, 53, 54, 112, 126, 65, 136, 138, 148, 150, 3, 162, 83, 172, 89, 90, 95, 24, 196, 66, 14, 222, 113, 114, 8, 119, 120, 125, 256, 131, 268, 54, 138, 280, ... (sequence A246489 in the OEIS)

Smallest prime with duodecimal period n are (in base 10)

11, 13, 157, 5, 22621, 7, 659, 89, 37, 19141, 23, 20593, 477517, 211, 61, 17, 2693651, 1657, 29043636306420266077, 85403261, 8177824843189, 57154490053, 47, 193, 303551, 79, 306829, 673, 59, 31, 373, 153953, 886381, 2551, 71, 73, ... (sequence A252170 in the OEIS)

Irrational numbers[]

The representations of irrational numbers in any positional number system (including decimal and duodecimal) neither terminate nor repeat. The following table gives the first digits for some important algebraic and transcendental numbers in both decimal and duodecimal.

Algebraic irrational number	In decimal	In duodecimal
√2, the square root of 2	1.414213562373...	1;4B79170A07B8...
$φ$ (phi), the golden ratio = ${\tfrac {1+{\sqrt {5}}}{2}}$	1.618033988749...	1;74BB6772802A...
Transcendental number	In decimal	In duodecimal
$π$ (pi), the ratio of a circle's circumference to its diameter	3.141592653589...	3;184809493B91...
$e$ , the base of the natural logarithm	2.718281828459...	2;875236069821...

References[]

^ George Dvorsky (2013-01-18). "Why We Should Switch To A Base-12 Counting System". Archived from the original on 2013-01-21. Retrieved 2013-12-21.
^ Matsushita, Shuji (1998). Decimal vs. Duodecimal: An interaction between two systems of numeration. 2nd Meeting of the AFLANG, October 1998, Tokyo. Archived from the original on 2008-10-05. Retrieved 2011-05-29.
^ Mazaudon, Martine (2002). "Les principes de construction du nombre dans les langues tibéto-birmanes". In François, Jacques (ed.). La Pluralité (PDF). Leuven: Peeters. pp. 91–119. ISBN 90-429-1295-2.
^ von Mengden, Ferdinand (2006). "The peculiarities of the Old English numeral system". In Nikolaus Ritt; Herbert Schendl; Christiane Dalton-Puffer; Dieter Kastovsky (eds.). Medieval English and its Heritage: Structure Meaning and Mechanisms of Change. Studies in English Medieval Language and Literature. Vol. 16. Frankfurt: Peter Lang. pp. 125–145.
^ von Mengden, Ferdinand (2010). Cardinal Numerals: Old English from a Cross-Linguistic Perspective. Topics in English Linguistics. Vol. 67. Berlin; New York: De Gruyter Mouton. pp. 159–161.
^ Gordon, E V (1957). Introduction to Old Norse. Oxford: Claredon Press. pp. 292–293.
^ Pittman, Richard (1990). "Origin of Mesopotamian duodecimal and sexagesimal counting systems". Philippine Journal of Linguistics. 21 (1): 97.
^ Nishikawa, Yoshiaki (2002). "ヒマラヤの満月と十二進法" [The Full Moon in the Himalayas and the Duodecimal System] (in Japanese). Archived from the original on March 29, 2008. Retrieved 2008-03-24.
^ Ifrah, Georges (2000). The Universal History of Numbers: From prehistory to the invention of the computer. John Wiley and Sons. ISBN 0-471-39340-1. Translated from the French by David Bellos, E.F. Harding, Sophie Wood and Ian Monk.
^ Macey, Samuel L. (1989). The Dynamics of Progress: Time, Method, and Measure. Atlanta, Georgia: University of Georgia Press. p. 92. ISBN 978-0-8203-3796-8.
^ ^a ^b ^c ^d ^e ^f ^g ^h De Vlieger, Michael (2010). "Symbology Overview" (PDF). The Duodecimal Bulletin. 4X [58] (2).
^ ^a ^b Andrews, Frank Emerson (1935). New Numbers: How Acceptance of a Duodecimal (12) Base Would Simplify Mathematics. p. 52.
^ "Annual Meeting of 1973 and Meeting of the Board" (PDF). The Duodecimal Bulletin. 25 [29] (1). 1974.
^ De Vlieger, Michael (2008). "Going Classic" (PDF). The Duodecimal Bulletin. 49 [57] (2).
^ ^a ^b "Mo for Megro" (PDF). The Duodecimal Bulletin. 1 (1). 1945.
^ Pitman, Isaac (ed.): A triple (twelve gross) Gems of Wisdom. London 1860
^ Pitman, Isaac (1947). "A Reckoning Reform [reprint from 1857]" (PDF). The Duodecimal Bulletin. 3 (2).
^ Karl Pentzlin (2013-03-30). "Proposal to encode Duodecimal Digit Forms in the UCS" (PDF). ISO/IEC JTC1/SC2/WG2, Document N4399. Retrieved 2016-05-30.
^ "The Unicode Standard, Version 8.0: Number Forms" (PDF). Unicode Consortium. Retrieved 2016-05-30.
^ "The Unicode Standard 8.0" (PDF). Retrieved 2014-07-18.
^ Scott Pakin (2009). "The Comprehensive LATEX Symbol List" (PDF). Retrieved 2016-05-30.
^ "What should the DSA do about transdecimal characters?". The Dozenal Society of America. Retrieved 2018-01-01.
^ ^a ^b Volan, John (July 2015). "Base Annotation Schemes" (PDF). Duodecomal Bulletin. 62.
^ ^a ^b Zirkel, Gene (2010). "How Do You Pronounce Dozenals?" (PDF). The Duodecimal Bulletin. 4E [59] (2).
^ "Systematic Dozenal Nomenclature and other nomenclature systems" (PDF). The Duodecimal Bulletin. Retrieved 2019-07-28.
^ ^a ^b ^c Goodman, Donald (2016). "Manual of the Dozenal System" (PDF). Dozenal Society of America. Retrieved 27 April 2018.
^ The Prodigy (Biography of WJS) pg [42]
^ William S. Crosby; "The Uncial Jottings of a Harried Infantryman", The Duodecimal Bulletin, Vol 1 Issue 2, June 1945, Page 9.
^ A. C. Aitken (January 25, 1962) "Twelves and Tens" The Listener.
^ A. C. Aitken (1962) The Case Against Decimalisation. Edinburgh / London: Oliver & Boyd.
^ "SchoolhouseRock - Little Twelvetoes". 6 February 2010. Archived from the original on 6 February 2010.
^ Bellos, Alex (2011-04-04). Alex's Adventures in Numberland. A&C Black. p. 50. ISBN 978-1-4088-0959-4.
^ Pendlebury, Tom; Goodman, Donald (2012). "TGM: A Coherent Dozenal Metrology" (PDF). The Dozenal Society of Great Britain.
^ Suga, Takashi (22 May 2019). "Proposal for the Universal Unit System" (PDF).
^ Volan, John. "The Primel Metrology" (PDF). The Duodecimal Bulletin. 63 (1): 38–60. Retrieved 30 July 2021.
^ Michael Thomas De Vlieger (30 November 2011). "Dozenal FAQs" (PDF). The Dozenal Society of America.

External links[]

[io9-1] George Dvorsky (2013-01-18). "Why We Should Switch To A Base-12 Counting System". Archived from the original on 2013-01-21. Retrieved 2013-12-21.

[2] Matsushita, Shuji (1998). Decimal vs. Duodecimal: An interaction between two systems of numeration. 2nd Meeting of the AFLANG, October 1998, Tokyo. Archived from the original on 2008-10-05. Retrieved 2011-05-29.

[3] Mazaudon, Martine (2002). "Les principes de construction du nombre dans les langues tibéto-birmanes". In François, Jacques (ed.). La Pluralité (PDF). Leuven: Peeters. pp. 91–119. ISBN 90-429-1295-2.

[4] von Mengden, Ferdinand (2006). "The peculiarities of the Old English numeral system". In Nikolaus Ritt; Herbert Schendl; Christiane Dalton-Puffer; Dieter Kastovsky (eds.). Medieval English and its Heritage: Structure Meaning and Mechanisms of Change. Studies in English Medieval Language and Literature. Vol. 16. Frankfurt: Peter Lang. pp. 125–145.

[5] von Mengden, Ferdinand (2010). Cardinal Numerals: Old English from a Cross-Linguistic Perspective. Topics in English Linguistics. Vol. 67. Berlin; New York: De Gruyter Mouton. pp. 159–161.

[6] Gordon, E V (1957). Introduction to Old Norse. Oxford: Claredon Press. pp. 292–293.

[7] Pittman, Richard (1990). "Origin of Mesopotamian duodecimal and sexagesimal counting systems". Philippine Journal of Linguistics. 21 (1): 97.

[8] Nishikawa, Yoshiaki (2002). "ヒマラヤの満月と十二進法" [The Full Moon in the Himalayas and the Duodecimal System] (in Japanese). Archived from the original on March 29, 2008. Retrieved 2008-03-24.

[Ifrah-9] Ifrah, Georges (2000). The Universal History of Numbers: From prehistory to the invention of the computer. John Wiley and Sons. ISBN 0-471-39340-1. Translated from the French by David Bellos, E.F. Harding, Sophie Wood and Ian Monk.

[Macey-10] Macey, Samuel L. (1989). The Dynamics of Progress: Time, Method, and Measure. Atlanta, Georgia: University of Georgia Press. p. 92. ISBN 978-0-8203-3796-8.

[Symbology_Overview-11] ^ ^a ^b ^c ^d ^e ^f ^g ^h De Vlieger, Michael (2010). "Symbology Overview" (PDF). The Duodecimal Bulletin. 4X [58] (2).

[New_Numbers_1935-12] Andrews, Frank Emerson (1935). New Numbers: How Acceptance of a Duodecimal (12) Base Would Simplify Mathematics. p. 52.

[13] "Annual Meeting of 1973 and Meeting of the Board" (PDF). The Duodecimal Bulletin. 25 [29] (1). 1974.

[14] De Vlieger, Michael (2008). "Going Classic" (PDF). The Duodecimal Bulletin. 49 [57] (2).

[DB01-15] "Mo for Megro" (PDF). The Duodecimal Bulletin. 1 (1). 1945.

[16] Pitman, Isaac (ed.): A triple (twelve gross) Gems of Wisdom. London 1860

[Pitman1857-17] Pitman, Isaac (1947). "A Reckoning Reform [reprint from 1857]" (PDF). The Duodecimal Bulletin. 3 (2).

[N4399-18] Karl Pentzlin (2013-03-30). "Proposal to encode Duodecimal Digit Forms in the UCS" (PDF). ISO/IEC JTC1/SC2/WG2, Document N4399. Retrieved 2016-05-30.

[Unicode8-19] "The Unicode Standard, Version 8.0: Number Forms" (PDF). Unicode Consortium. Retrieved 2016-05-30.

[20] "The Unicode Standard 8.0" (PDF). Retrieved 2014-07-18.

[LATEX-21] Scott Pakin (2009). "The Comprehensive LATEX Symbol List" (PDF). Retrieved 2016-05-30.

[22] "What should the DSA do about transdecimal characters?". The Dozenal Society of America. Retrieved 2018-01-01.

[:1-23] Volan, John (July 2015). "Base Annotation Schemes" (PDF). Duodecomal Bulletin. 62.

[Zirkel2010-24] Zirkel, Gene (2010). "How Do You Pronounce Dozenals?" (PDF). The Duodecimal Bulletin. 4E [59] (2).

[DBX1-25] "Systematic Dozenal Nomenclature and other nomenclature systems" (PDF). The Duodecimal Bulletin. Retrieved 2019-07-28.

[DSAGoodman2016-26] Goodman, Donald (2016). "Manual of the Dozenal System" (PDF). Dozenal Society of America. Retrieved 27 April 2018.

[27] The Prodigy (Biography of WJS) pg [42]

[28] William S. Crosby; "The Uncial Jottings of a Harried Infantryman", The Duodecimal Bulletin, Vol 1 Issue 2, June 1945, Page 9.

[29] A. C. Aitken (January 25, 1962) "Twelves and Tens" The Listener.

[30] A. C. Aitken (1962) The Case Against Decimalisation. Edinburgh / London: Oliver & Boyd.

[31] "SchoolhouseRock - Little Twelvetoes". 6 February 2010. Archived from the original on 6 February 2010.

[32] Bellos, Alex (2011-04-04). Alex's Adventures in Numberland. A&C Black. p. 50. ISBN 978-1-4088-0959-4.

[33] Pendlebury, Tom; Goodman, Donald (2012). "TGM: A Coherent Dozenal Metrology" (PDF). The Dozenal Society of Great Britain.

[34] Suga, Takashi (22 May 2019). "Proposal for the Universal Unit System" (PDF).

[35] Volan, John. "The Primel Metrology" (PDF). The Duodecimal Bulletin. 63 (1): 38–60. Retrieved 30 July 2021.

[36] Michael Thomas De Vlieger (30 November 2011). "Dozenal FAQs" (PDF). The Dozenal Society of America.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]