At one time, people looked upon the baker's profession with great suspicion. In the bakery business it was easy to sell one's customers short on weight. That is, to not include as much bread (in terms of weight) as was promised. To compensate for this, a British law was passed that provided severe penalties for bakers caught cheating their customers. The laws basically set forth regulations for standard weights of bread.

This seems to have scared many bakers, for to be on the safe side, they started giving individual customers an extra piece with each piece purchased and wholesalers (who would buy loaves by the dozen) would be given an extra loaf. That was the origin of giving 13 instead of 12, and in time, this came to be referred to as a "baker's dozen."