A baker's dozen or long dozen is a collection of thirteen items, usually baked goods such as bread, rolls, and pastries. The term “baker's dozen” has its roots in the 13th century, and a number of theories have been put forward to explain its origins. The most likely explanation for the baker's dozen is related to the severe punishments which existed in England for bakers who shorted their customers; out of fear of accidentally violating the law, bakers threw an extra loaf in to make sure that the lot of bread would be of the proper weight.
Most bakers in medieval England were members of the Worshipful Company of Bakers, a trade guild which regulated bread prices, the various types of bread available for sale, and where bakers could establish new shops. Members of the guild paid an annual fee, and the guild itself submitted payments to the Treasury of England so that it would be recognized as an official guild. The first such payment was made in 1155, making the Worshipful Company of Bakers one of the oldest trade associations in England.
In 1266, Henry III proclaimed an Assize of Bread and Ale, a law relating to the sale of grains. According to the Assize, a lot of bread had to meet a particular weight requirement to be sold. This law was enacted because bakers had a habit of shorting their clients, which may have been accidental due to the unreliable measuring systems of the period. However, the punishment for violation could include the loss of a hand, and bakers began including an additional loaf to ensure that they complied with the Assize, creating a baker's dozen. This type of long measure can be found in other trades as well, and might be considered a sort of insurance against customer complaint.
Another theory for the origins of a baker's dozen is that a baker selling bread to a retailer might offer a 13th loaf for the retailer's cut of the profits. Yet another theory suggests that bakers made a baker's dozen to ensure that there was an even dozen in case a loaf became damaged or spoiled. This seems less likely, as most commercial bakeries make large batches of baked goods, rather than individual lots of one dozen.
Most members of the English speaking world recognize a baker's dozen as an assortment of 13 baked goods, and some bakeries still sell lots of 13 items. Most baked goods today are sold on a per item basis, rather than a weight basis, as even sophisticated measuring machines can make mistakes, especially with an unpredictable baked good like bread. Bakers will no longer lose a hand on the basis of a short measure, but the baker's dozen has entered tradition, even if the original cause for the phrase has been forgotten.