 In analyses of the sort Larson exemplifies, the structure of an expression is given by a derivation, which consists of a sequence of successively modified trees. Larson calls the earliest ones underlying structures. The last (and least abstract) in the derivation is the surface structure, which captures properties relevant to the way the expression is written and pronounced. The underlying structures are posited in order to better identify syntactic generalizations. They are related to surface structures by a series of operations called transformations (which generative Essentialists typically regard as mentally real operations of the human language faculty).