How is a baker's dozen different from a standard dozen?

A dozen is a unit of quantity, equal to 12. A baker’s dozen is an informal unit of quantity, equal to 13. Bakers often toss in an extra item for each dozen bought, making a total of 13. This custom is very old, dating at least from the thirteenth century, when the weights and prices of loaves of bread were strictly regulated by royal proclamations called assizes, and bakers could be jailed if they failed to provide fair weight at the listed prices.