We all (I hope) know that a dozen means twelve of something, so why do bakers use the number 13 when referring to a dozen? The baker’s dozen dates back to ancient times and laws.
Way back in history people were afraid that the baker might cheat them, by giving them a “light loaf” or perhaps a smaller loaf than usual. So laws were enacted to protect the consumer and severely punish the baker if this were to happen. In ancient Egypt, if a baker was caught cheating a customer his ear would be nailed to the door of his bakery, I can only assume they cut it off first. In Babylon, a baker could have his hand cut off for selling a “light loaf.”
Similarly, harsh laws could be found throughout Europe, and given the intricacies of baking bread, it wasn’t hard for a baker to accidentally give a customer bread that was a little lighter or smaller than the day before. Thus, the baker’s dozen was born.
Baker’s obviously didn’t want their hands cut off or some other equally gruesome punishment bestowed on them, so in order to protect themselves, they began to add 1 extra to every dozen of anything a person bought. This little bit extra insured the baker that customers couldn’t claim that their bread was underweight or too small as there was another loaf or scoop to make up the difference. Thus protecting bakers and giving us the baker’s dozen.