A term used in linguistics to refer to an abstract level of representation of a sentence postulated in order to explain the patterns encountered in the empirical data of a language. The notion of underlying representation or underlying forms is central to generative grammar, where a stage of underlying structure is recognized in the derivation of sentences. In early transformational grammar, the underlying phrase-marker refers to the structural description of a sentence which is the result of the phrase-structure rules; this underlying string then acts as the input to the transformational rules, which thereby produce ‘derived’ phrase-markers. Later, the term deep structure came to be used as a specific conception of underlying structure, in the context of Aspects of the Theory of Syntax, from which surface structures are transformationally derived. In government-binding theory, the term D-structure is used. The extent to which the various underlying representations of sentences have psychological reality has been and remains controversial.