A formal grammatical system is described in which the lexicon plays a central role. A distinguishing feature of this system is its departure from the priniciple of monocategorial lexical attachment, that is, it permits many-one translations by the lexical attachment component from syntactic categories to morphemes. In this system the underlying syntactic and semantic representations of an expression are the same linguistic entity: a base tree prior to lexical attachment. Thus the base component generates an underlying language which has immediate semantic as well as syntactic significance. It is shown how such devices as rule features and spelling-out rules can be dispensed with in this framework. A lexical attachment criterion is formulated and a principle of disjunctive ordering is invoked as a constraint on the attachment of lexical entries. The various ways in which derived sematic/syntactic subtrees generated by the base component can be alternatively manifested by lexical items are discussed, and a simplicity criterion for lexical structure is proposed. Finally, it is shown how lexical treatment of word extension is to be preferred over transformational treatment of it. (Author)