Theoretical linguistics posits the existence of the null pronoun PRO as the theoretical basis for the analysis of control structures. The null pronoun PRO is an element that impacts a sentence in a similar manner to how a normal pronoun impacts a sentence, but the null pronoun is inaudible. The null PRO is added to the predicate, where it occupies the position that one would typically associate with an overt subject (if one were present). The following trees illustrate PRO in both constituency-based structures of phrase structure grammars and dependency-based structures of dependency grammars: