Sentence:  Around the thirteenth century, bakes began to add an extra loaf to every dozen. They did not want to be accused of cheating their customers. So 13 items has become known as a <WSD>baker's dozen</WSD>. ... Today, some bakers still bake bread in "<WSD>baker's dozens</WSD>." 
Meaning of the word marked with <WSD> tags:  Thirteen; a group of thirteen. (from 1590s)