Why does a baker's dozen have thirteen instead of twelve?

It started in the 13th century in England. If bakers shortchanged their customers they could be punished severely. (For example: If the bakery did not weigh enough, the baker could have his hand chopped off. They would also have a damaged reputation, and be known as a cheat.) Bakers began giving customers 13 for the price of 12 to be certain this would not happen to them.