in generative linguistics, pro (called "big pro", distinct from "pro", "small pro" or "little pro") is a pronominal determiner phrase (dp) without phonological content. as such, it is part of the set of empty categories. the null pronoun pro is postulated in the subject position of non-finite clauses. one property of pro is that, when it occurs in a non-finite complement clause, it can be bound by the main clause subject ("subject control") or the main clause object ("object control"). the presence of pro in non-finite clauses lacking overt subjects allows a principled solution for problems relating to binding theory.