The popular tale behind this phrase’s origin is that a medieval law specified the weight of loaves of bread and any baker who shorted a customer was in for dire punishment. So, bakers would include a thirteenth loaf with each dozen just to be safe. The story is partly true. There was such a law, but the practice of adding an extra loaf to the dozen had nothing to do with fear of punishment.
The law in question was the Assize of Bread and Ale, first promulgated in England in 1266. There are various versions of the law promulgated over the years, but they all regulated the size and price of loaves of bread that were sold on the market. During years of good harvests, bakers could make more bread than they could sell locally, so they would sell the excess loaves to hucksters, or middlemen. But since the size and price were strictly regulated, the only way for these distributors to make money would be for the baker to give them extra loaves. The baker would give the huckster a thirteenth, or vantage, loaf for each dozen. This extra loaf provided the profit for the middleman.
The practice of adding the thirteenth loaf is older than the phrase. The phrase only dates to 1599.
Copyright 1997-2016, by David Wilton