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.