A dozen (commonly abbreviated doz or dz) is a grouping of 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. Twelve is convenient because it has more divisors than other small numbers: 12 = 2 × 6 = 3 × 4 = 1 × 12. The use of twelve as a base number, known as the duodecimal system (also as dozenal), probably originated in Mesopotamia (see also sexagesimal). This could come from counting on one's fingers by counting each finger bone with one's thumb. Using this method, one hand can count to twelve, and two hands can count to 144. 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 baker's dozen, also known as a big or long dozen, is 13.