A dozen consists of twelve items, but a baker's dozen usually has thirteen.

Today's offer at the pastry shop: you buy twelve pastries and you get a baker's dozen, which means you get an extra one for free

L'offerta di oggi in pasticceria: compri 12 pasticcini e ne ricevi uno gratis, cioè 13 pasticcini

For S. Valentine's day our flower-shop has a special “dozen” for sale: not 12, like it should be; not 13, like a baker's dozen, but 14 red roses for the price of 12

Per S. Valentino il nostro fiorista ha una speciale “dozzina” in offerta: non 12, come dovrebbe essere, e nemmeno 13, ma 14 rose rosse al prezzo di 12

In 1266, the English Parliament passed a law that specified exactly how much a loaf of bread should weigh, and imposed a heavy penalty for short weight. To protect themselves, bakers would give their customers 13 loaves instead of 12, and in the 16th century this came to be called “a baker's dozen”