The Yoruba of West Africa have a number system based on counting cowries, shells used as currency for millennia. Up to the number 400, Yoruba numbers are organized by twenties, an organization typically based on counting with the fingers and toes. Starting with 200, Yoruba numbers are organized by tens, suggesting a historically recent expansion based on counting bundles of cowrie shells in conjunction with selling slaves to European traders. Yoruba numbers involve more than a typical amount of subtractive formations, an operation known as back-counting. They are also curiously flexible in how they can be formed, permitting any particular number to be expressed in an unusual number of alternative but well-formed ways. A typical example is 19,669, which can be expressed in 11 different ways. This flexibility suggests that Yoruba numbers are conceived of as arrays of combinatorial possibilities, rather than as points along a linear continuum as in the Western number line. An array of cowrie shells has also been described as being used like an abacus, and this visual arrangement may help influence these numbers to be conceptualized as arrays.