In linguistics, and especially the study of syntax, the deep structure of a linguistic expression is a theoretical construct that seeks to unify several related observed forms. For example, the sentences "Pat loves Chris" and "Chris is loved by Pat" mean roughly the same thing and use similar words. Some linguists, in particular Noam Chomsky, have tried to account for this similarity by positing that these two sentences are distinct surface forms that were derived from an unobservable common source, the so-called deep structure underlying both sentences.