In nineteenth-century America and the Atlantic world, the "rule of three" was usually regarded as the endpoint of a basic mathematics education. This essay considers the importance of the rule as a technology that enabled broader access to the calculations necessary to participate in the increasingly global market economy. Used by workmen, women, and even the enslaved, the rule and related tools translated basic literacy into practical numeracy. By doing so, it offered a diverse range of people the ability to negotiate more effectively. At the same time, however, the rule's spread helped to legitimate particular types of exchange and commensuration, and with them the emerging capitalist economy.