A set of twelve items. In the bakery trade a dozen is supplied as thirteen. Known as a ‘baker’s dozen’, this is a reminder of the days when the substantial penalties under the law for giving short weight in bread caused bakers to add an extra loaf to a dozen ordered so as to be on the safe side of error. There is also a tradition in the newspaper trade of supplying copies to retailers at thirteen to the dozen invoiced. Where unit costs are very low such a practice can be costeffective in so far as it minimizes time spent dealing with queries on shortages in the number supplied.
Reference: The Penguin Business Dictionary, 3rd edt.