Quote from : Harry Potter and the Order of the Phoenix Book

Okay, I've only just found out the final lineup for Slytherin," said Angelina, consulting a piece of parchment. "Last year's Beaters, Derrick and Bole, have left now, but it looks as though Montague's replaced them with the usual gorillas, rather than anyone who can fly particularly well. They're two blokes called Crabbe and Goyle. I don't know much about them--" "We do," said Harry and Ron together. "Well they don't look bright enough to tell one end of a broom from another," said Angelina, pocketing her parchment, "but then I was always surprised Derrick and Bole managed to find their way onto the pitch without signposts." "Crabbe and Goyle are in the same mold," Harry assured her.