Why *if or not but ✓ whether or not

This squib provides an account of a contrast between whether and if in English, manifested in the contrast between the grammaticality of I don’t know whether or not Pat will arrive and the ungrammaticality of *I don’t know if or not Pat will arrive. I argue that this contrast can be explained if we assume that whether can pied-pipe, but there is no pied-piping in if-questions. Strikingly, once the pied-piping parse for whether is eliminated, it behaves like if. Then I show that this contrast exists crosslinguistically: Polish alternative questions behave like whether-questions because pied-piping is possible, and Bengali alternative questions behave like if-questions because pied-piping is not possible.