In the United Kingdom when selling certain goods, bakers were obliged to sell goods by the dozen at a specific weight or quality (or a specific average weight). During this time, bakers who sold a dozen units that failed to meet this requirement could be penalized with a fine. Therefore to avoid risking this penalty, some bakers included an extra unit to be sure the minimum weight was met, bringing the total to 13 units or what is now commonly known as a baker's dozen.