How many items make up a baker's dozen?
A dozen (commonly abbreviated doz or dz) is a grouping of twelve. A baker's dozen, devil's dozen, long dozen, or long measure is 13, one more than a standard dozen. The practice of baking 13 items for an intended dozen was insurance against the items being lower than the statutory weight, or of lower than usual quality, which could cause the baker to be fined.
According to the 1811 Dictionary of the Vulgar Tongue, by Captain Grose, "a Baker's Dozen is Thirteen; that number of rolls being allowed to the purchaser of a dozen".