Why bakers dozen used?

During times of high corn prices, bakers were suspected of cheating customers by putting less rolls or buns in a bag but charging for a full dozen. In order to stamp out this practice, stiff penalties were passed on bakers shortchanging their customers.

In order to avoid accidentally putting less bread in a bag and falling foul of this law, bakers took to sticking an extra bun in the bag so that even if they miscounted, they wouldn't be punished.

So 'a baker's dozen' means thirteen!