Baker's Dozen

 
A dozen is another name for twelve. But a baker's dozen is thirteen. The reason for this goes back in history to the days when bakers were heavily fined if they gave short measure. To ensure they weren't caught out the baker would give a surplus numnber of loaves - called the inbread - to make sure they ran no risk of a fine or worse penalty. The thirteenth loaf was called the vantage loaf.