Dozen is another word for the number twelve. The dozen may be one of the earliest primitive groupings, perhaps because there are approximately a dozen cycles of the moon or months in a cycle of the sun or year. The dozen is convenient because its multiples and divisors are convenient: 12 = 2 × 2 × 3, 3 × 4 = 2 × 6, 60 = 12 × 5, 360 = 12 × 30. The use of twelve as a base number, known as the duodecimal system (also as dozenal), probably originated in Mesopotamia (see also sexagesimal). Twelve dozen (122 = 144, the duodecimal 100) are known as a gross; and twelve gross (123 = 1,728, the duodecimal 1,000) are called a great gross, a term most often used when shipping or buying items in bulk. A great hundred, also known as a small gross, is 120 or ten dozen (a dozen for each finger on both hands). A baker's dozen, also known as a long dozen, is thirteen.