A Baker's Dozen is not 12 but 13 and there are two established theories as to the phrase’s origin. The first theory involves medieval England and identifies the sales techniques of tradesmen such as fishmongers and bakers. When selling straight to the public, sellers would have a fixed price for their wares. But when selling to bulk buyers, such as market stallholders, the trader would gift them an extra item for every dozen bought. In such cases the 13th loaf or fish would be the stallholder's profit on the items he buys and then sells at the market.

The second hypothesis dates back to the 1200s, when bakers had the reputation of selling underweight loaves – sometimes unfairly because the baking process sometimes made the bread “thin” on the inside. In 1266 rules were introduced to standardise weights of various loaves and the penalty for being underweight could mean a day in the stocks. To steer clear of this, bakers began to add an extra loaf, known as a “vantage loaf”, to every dozen sold, to make certain they stayed within the new laws.