baker's dozen, n.All

A Baker's dozen is a dozen plus one for the devil to spoil

A baker's dozen is 13 because of the "assisa panis" law enacted in 1266 by Henry III, which established harsh fines for bakers who sold underweight bread. They threw in one extra just to be safe.

A Baker's dozen is 13 on account of the fact that bakers often miscount when hurried. Thus, they add a buffer to ensure that if they've undercounted, they will not be accused of cheating the customer

A Baker's dozen is a dozen zero-indexed