obligatory (adj.)

A term in LINGUISTICS which refers to an ELEMENT that cannot be removed from a STRUCTURE without that structure becoming UNGRAMMATICAL. In the sentence I put the pen in the desk, in the desk is an obligatory ADVERBIAL. In early TRANSFORMATIONAL grammar, the term refers to one of the two types of TRANSFORMATIONAL RULE postulated by Noam Chomsky in his book Syntactic Structures (1957), the other being OPTIONAL. An obligatory transformation is one which must apply at a given stage in a DERIVATION, when its STRUCTURAL DESCRIPTION is met, if a WELL-FORMED SENTENCE is to result, e.g. the rule which attaches AFFIXES to their BASE forms. In later versions of transformational grammar, the range of this notion changes as SURFACE STRUCTURES come to be derived from DEEP STRUCTURES by obligatory transformations, and the notion of optional selection comes to be replaced by choices made between the rules of the base COMPONENT.