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There are several uses of 12 in some old systems of measurement. Some of them make sense given current context (There are 12 lunar cycles per year), however some of them seem to be arbitrarily chosen. Just to name a few...

  • There are 12 inches in a foot
  • Day and night are each divided into 12 hours
  • There are 12 signs in both Western and Chinese Zodiacs
  • We have a word specifically for twelve (dozen)
  • We have a word for 12 12's (gross)
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  • $\begingroup$ As Conifold's answer demonstrates, we know a lot about the origins of some of the examples use mentioned. However, as far as I know, considerably less is known about the early origins and development of English units of length such as the inch and foot. $\endgroup$ – David H Oct 6 '15 at 23:51
  • $\begingroup$ 12 is useful because it can be divided so easily (by 6,4,3,2) 10 is just as arbitrary and a lot less convenient $\endgroup$ – Martin Beckett Oct 7 '15 at 21:41
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The duodecimal system as a developed positional system of numerals does not appear historically. However, ancient Egyptians, Babylonians and Chinese used duodecimal units in astronomical and time keeping matters. From Scientific American:

"As early as 1500 B.C., the Egyptians had developed a more advanced sundial. A T-shaped bar placed in the ground, this instrument was calibrated to divide the interval between sunrise and sunset into 12 parts. This division reflected Egypt's use of the duodecimal system--the importance of the number 12 is typically attributed either to the fact that it equals the number of lunar cycles in a year or the number of finger joints on each hand (three in each of the four fingers, excluding the thumb), making it possible to count to 12 with the thumb...

During the era when sundials were first used, however, Egyptian astronomers also first observed a set of 36 stars that divided the circle of the heavens into equal parts. The passage of night could be marked by the appearance of 18 of these stars, three of which were assigned to each of the two twilight periods when the stars were difficult to view. The period of total darkness was marked by the remaining 12 stars, again resulting in 12 divisions of night (another nod to the duodecimal system). During the New Kingdom (1550 to 1070 B.C.), this measuring system was simplified to use a set of 24 stars, 12 of which marked the passage of the night... Once both the light and dark hours were divided into 12 parts, the concept of a 24-hour day was in place".

Modern fixed-length hours, however, were only first suggested by Hipparchus c.140 BC, who used their length on the days of equinox. Seasonally varying hours remained in use for many centuries hence.

Babylonians also originally had twelve divisions in a day, eventually increased to modern 24. Traditional Chinese calendars, clocks, and compasses are based on the twelve Earthly Branches, a calendar system derived from the motion of Suìxīng, the Year Star, our Jupiter. The celestial circle was divided into 12 sections, and the orbital period was rounded from 11.86 to 12 years. The duodecimal weight units go back to Romans, Latin uncia gave birth to modern English ounce and inch.

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  • $\begingroup$ Do you have a reference for "Babylonians also originally had twelve divisions in a day"? In absence of evidence I would suspect that they also divided the night into 12 hours. $\endgroup$ – user2255 Oct 7 '15 at 17:59

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