Some items typically sold or grouped in a set of 12, also known as a dozen, include eggs, flowers and doughnuts. Because the number 12 is easily divisible into halves, thirds and quarters, many manufacturers package commonly used items, such as pencils and disposable cups, by the dozen.
The ancient Mesopotamians were likely the first people to use 12 as a unit of measurement. The concept probably developed because each human finger has three joints, making it possible to count to 12 on one hand using the thumb. The number 12 also had symbolic significance in the ancient world, because each year consists of approximately 12 lunar cycles.
Items packaged by the dozen can also be grouped together in larger sets, a common practice when manufacturers sell items in bulk. Twelve dozens make up a gross, while 12 gross make up a great gross.
Baked goods are sometimes sold in a unit called a "baker's dozen," which is a set of 13 items. The practice of including an extra item came about in the 13th century when the penalty for cheating customers was amputation of a hand. To avoid the risk of losing a hand due to imprecise measurements, bakers included an extra piece in each dozen.