One reason is that the metric system is based on the number 10. The metric system emerged in the late eighteenth century out of a need to bring standardization to measurement. But 10 was important before the metric system. For example, Nichomachus of Gerasa, a second-century mathematician from Judea, considered 10 a perfect number, the figure of divinity present in creation with humankind’s fingers and toes. Pythagoreans believed 10 to be “the first-born of the numbers, the mother of them all, the one that never wavers and gives the key to all things.”

And shepherds of West Africa counted sheep in their flocks by colored shells based on 10, and 10 had evolved as a “base” of most numbering schemes. Some historians believe the reason 10 developed as a base number had more to do with ease: 10 is easily counted on figures and the rules of addition, subtraction, multiplication, and division for the number 10 are easily memorized.