A 'baker's dozen' refers to the number 13, and the practice originated from strict English laws regarding the weight of bread. The 'Assize of Bread and Ale' was a law that heavily penalized bakers who sold short-weighted loaves. Since bread can vary in weight due to air bubbles or moisture loss during baking, bakers lived in fear of being fined or even whipped for accidentally selling a loaf that was too light. To protect themselves, when a customer ordered 12 loaves (a dozen), the baker would throw in a 13th loaf for free to ensure the total weight exceeded the legal requirement. This everyday safety margin became a lasting tradition that survives in many bakeries today.