from The Century Dictionary and Cyclopedia
- n. (noun) The thirteenth loaf in a baker's dozen.
To guard against severe punishments, bakers included an extra loaf of bread per every dozen. This was also called the 'vantage loaf,' because it gave the baker the 'vantage' (that is, the advantage) of not being punished, and the 'in bread,' because it was bread 'thrown in' with the order. Both of these terms date from the 1600s.
So bakers made it a practice to add to each order of bread the 13th loaf, the in bread or vantage loaf. This guaranteed there would be no short changing - or 'ear nailing' as it was often called.
A baker's dozen, or a group of 13, comes from the former custom among bakers of adding an extra roll as a safeguard against the possibility of 12 weighing light. The 'vantage-loaf' was also referred to as the 'in-bread.'