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Duodecimal is a base 12 numeral system.
Since 2, 3, 4, 6 are factors of 12, it is a more convenient number system for computing fractions. compared to decimal system which has only the factors 2 and 5.
Use of the base 12 number system is not common, but at least one example of duodecimal numerals is in use in the Chepang language of Nepal. Historically, the Romans, although they counted in base ten, used a duodecimal system to represent fractions. The Chinese Earthly Branches number system was also base 12. In fiction, J.R.R. Tolkien's Elvish languages used a duodecimal system.
Historically, the number 12 was used in many civilizations. It is believed that the observation of 12 appearances of the Moon in a year is the reason this number is used universally regardless of culture. Example of such usage include 12 months in a year, 12 hours on a clock, 12 traditional time divisions in a Chinese day, 12 signs of the zodiac in horoscope, 12 animal signs in Chinese astrology, etc.
Many European languages have special words for 11 and 12 (and sometimes into the teens), which are often misinterpreted as vestiges of a base 12 system. However, in actuality, most, if not all, are derived from decimal roots. For example, in Latin, the teens were formed by suffixing -decem (ten) to the respective words. In the modern Romance languages, this is often obscured by sound changes. For example, undecem and duodecem became, in Spanish, once and doce (likewise trece, catorce, quince). English "eleven" and "twelve" are believed to come from Proto-Germanic *ainlif and *twalif (respectively "one left" and "two left"), also related to base ten. Admittedly, the survival of such apparently unique terms may be connected with duodecimal tendencies, but their origin is not duodecimal.
Being a versatile denominator in fraction may explain why we have 12 inches in a foot, 12 ounces in a troy pound, 12 old British pence in a shilling, 12 items in a dozen, 12 dozens in a gross, 12 gross in a great gross, etc.
Decimal Equivalent 10 twelve (or a dozen) 12 100 one gross 12^2 = 144 1000 one great gross 12^3 = 1728 10 000 twelve great gross 12^4 = 20 736 100 000 ? 12^5 = 248 832 1 000 000 ? 12^6 = 2 985 984 15 a dozen and five 3B three dozen and eleven ABB ten gross eleven dozen and eleven 11B1 one great gross one gross eleven dozen and one (= the year 2005) 36 A17 three dozen and six great gross ten gross one dozen and seven Note that in English we say "a gross of apples", and not "a gross apples". In a hypothetical duo-decimal system, the term per gross (¹⁄144) might replace per cent (¹⁄100).
Fractions Duodecimal fractions are usually either very 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
or complicated (A = ten, B = eleven) - 1/5 = 0.24972497... recurring (easily rounded to 0.25)
- 1/7 = 0.186A35186A35... recurring
- 1/A = 0.124972497... recurring (rounded to 0.125)
- 1/B = 0.11111... recurring (rounded to 0.11)
- 1/11 = 0.0B0B... recurring (rounded to 0.0B)
As explained in recurring decimals, whenever a fraction is written in "decimal" 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 base-10 (= 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 (22×53) 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 base 10.
Arguably, factors of 3 are more commonly encountered in real-life division problems than factors of 5 (or would be, were it not for the decimal system having influenced our culture). Thus, in practical applications, the nuisance of recurring decimals is encountered less often when duodecimal notation is used. Advocates of duo-decimal 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 is between two prime numbers 11 and 13, whereas 10 is adjacent to composite number 9.
Advocacy and "dozenalism"
The case for the duodecimal system was put forth at length in F. 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 decimal-based weights and measure or by the adoption of the duodecimal number system. In contrary to the used symbols 'A' for ten and 'B' for eleven as used in hexadecimal notation, he suggested in his book and used a script X and a script E,  and  , to represent the digits ten and eleven respectively, because, at least on a page of Roman script, these characters were distinct from any existing letters or numerals, yet were readily available in printers' fonts. He chose for its resemblance to the Roman numeral X, and as the first letter of the word "eleven".
There are modern advocacy groups that promote the use of the duo-decimal system, and they sometimes use the word dozenal, rejecting the duo-decimal as a word based on the decimal counting system.
The Dozenal Society of America and Dozenal Society of Great Britain promote this base 12 system, arguing that it is better than the decimal system mathematically and in many other ways.
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