A

# twelve

12-string guitar.

Twelve is a number heavily used for grouping things (inches, hours, 12-packs), partly because it can be divided evenly in several different ways (by 2, 3, 4, and 6) and partly because there are roughly 12 cycles of the Moon for every one of the Sun. The Latin duodecim (two + ten) for twelve forms the root of dodecagon (originally duodecagon), meaning a 12-sided shape, and duodenum, the first part of the intestine that is about twelve inches long. Contracted and modified over the years, duodecim became "dozen." The number system based on 12 is called duodecimal.

Multiples of 12 have also been used by many cultures for various units and measures. A "shock" was 60 or five dozen (a dozen for each finger on one hand), and many cultures had a "great hundred" of 120 or ten dozen (a dozen for each finger on both hands). The Romans used a fraction system based on 12 and the smallest part, an uncil, became our word for an ounce. The French emperor Charlemagne established a monetary system that had a base of twelve and twenty, the remnants of which persist. Until 1970, the English pound sterling consisted of 20 shillings, and each shilling contained 12 pence. In 1944, The Duodecimal Society (which later changed its named to the Dozenal Society) was formed in New York with the purpose of proposing a switch to base 12 for all scientific work. There are 12 signs of the zodiac and there were 12 apostles of Christ.

Twelve is the smallest abundant number, a Niven number, and a semiperfect number (because 12 = 1 + 2 + 3 + 6; see perfect number).