Comrie's work is also noteworthy for having brought the notion of syntactic hierarchy to bear on the typology of causative constructions. A hierarchy of grammatical relations had already been formulated to help explain possibilities for relative clause formation (first presented as Keenan and Comrie's (1972) NP accessibility hierarchy; see Croft 1990: 147), and Comrie argued that a similar hierarchy was in play, at least in some constructions, in the marking of the original A argument when a base transitive clause is causativized. The hierarchy is as follows: