A “dozen” means twelve, so why does a “baker’s dozen” mean 13? Well, back in medieval England, bakers were small business owners who didn’t have access to accurate measuring equipment. They just used their eyes and their years of experience to estimate their size and weight of the loaves they baked. The problem was that it was against the law for bakers and other traders to sell goods that were under the indicated weight. If a baker sold a dozen loaves, and they turned out to be short of the required weight, their reputation could be ruined , or worse yet, they could be fined or banned from their trade if the customer in question was influential.

So, to protect themselves, when bakers sold a dozen loaves, they’d toss in an extra one, making a total of thirteen. This tradition stuck through the centuries, and even as accurate scales became available to bakers, they still kept giving their customers an extra loaf as an incentive to encourage loyalty.