Generative metrics is the collective term for three distinct theories of verse structure advanced between 1966 and 1977. Inspired largely by the example of Noam Chomsky's Syntactic Structures (1957) and Chomsky and Morris Halle's The Sound Pattern of English (1968), these theories aim principally at the formulation of explicit linguistic rules that will generate all possible well-formed instances of a given meter (e.g. iambic pentameter) and exclude any that are not well-formed. T.V.F. Brogan notes that of the three theories, "[a]ll three have undergone major revision, so that each exists in two versions, the revised version being preferable to the original in every case."