During this time, bakers who sold a dozen units that failed to meet this requirement could be penalized with a fine. Therefore, to avoid risking this penalty, some bakers included an extra unit to be sure the minimum weight was met, bringing the total to 13 units or what is now commonly known as a baker's dozen.

Request a dozen eggs from a farmer, a dozen steaks from a butcher, or a dozen pencils from a traveling office supplies salesman, and you will almost certainly receive 12 of your chosen item (counting errors do happen). But a baker's dozen is commonly understood to mean 13.

baker's half dozen (idiomatic, uncommon) Seven, a group of seven.

The origin is uncertain but it is believed to originate from Medieval England when bakers were subject to strict laws that related the price of bread to the price of the wheat used to make it. A dozen (12) loaves was a common order and to avoid accidentally shortchanging a customer a baker would throw in an extra loaf.