"Baa, Baa, Black Sheep" is an English nursery rhyme, the earliest surviving version of which dates from 1731. The words have changed little in two and a half centuries. Uncorroborated theories have been advanced to explain the meaning of the rhyme. These include that it is a complaint against Medieval English taxes on wool and that it is about the slave trade. In the twentieth century, it was a subject of controversies in debates about political correctness. It has been used in literature and popular culture as a metaphor and allusion.