In a study of English auxiliary verb usage, it is proposed that this category of verbs share a characteristic that explains some idiosyncracies: they do not show morphosyntactic inflectional irregularities. According to this account, the relationship between auxiliaries and full verbs is distant, and the morphosyntactic categories that auxiliaries clearly manifest are not inflectional, as in full verbs, but instead are lexically specified. This account makes sense of the ordering of auxiliaries, the double "-ing" constraint on progressives, idiosyncratic behavior of "being" and "having," and other properties. In the context of Generalized Phrase Structure Grammar, a formulation that also constitutes a partial definition of the class "auxiliary" is proposed. This formulation operates within a system of defaults and is distinct from that of Head-Driven Phrase Structure Grammar. A 44-item bibliography is included. (MSE)