In 1266, King Henry III of England appointed a legal body, the Assize of Bread and Ale, to regulate the sale of bread by quality, weight, and price relative to the cost of wheat.
Bakers who were caught violating these laws would be subject to fines, beatings, or even imprisonment. Out of the necessity to avoid these punishments, the baker’s dozen was born.
During these harsh times, whenever a baker sold 12 loaves, they would include an extra or end piece of bread as insurance that they hit the weight requirement.