Dozen
The dozen is convenient because its multipliers and multiples are convenient: 12 = 3 × 2 × 2, and 360 = 30 × 12. The French word douzaine means "about twelve" and is part of a family of words with similar meanings, e.g. vingtaine – "about 20"  centaine – "about 100"  etc. A baker's dozen, also known as a long dozen, is thirteen.