A term used in traditional prosody to denote the use of one kind of foot in place of the foot normally required by the metrical pattern of a verse line. In English verse, the kind of substitution most commonly referred to by prosodists is the replacement of the first iamb in an iambic line by a trochee; this ‘initial trochaic inversion’, as it is called, appears in Tennyson's line:Far on the ringing plains of windy Troy.The substitution of an anapaest for an iamb, or of a dactyl for a trochee, is called trisyllabic substitution, since it increases the number of syllables from two to three. The feet known as the spondee (●●) and the pyrrhic (○○) are sometimes invoked as substitute feet where stressed or unstressed syllables occur in pairs. Thus Keats's lineO for a beaker full of the warm Southshows, in addition to its initial trochaic inversion, a metrical variation at the end, which would be described in traditional prosody as the substitution of a pyrrhic and a spondee for the final two iambs. Some more modern theories of versification, however, have rejected the concept of the foot and along with it that of substitution, accounting for such metrical variations in terms of demotion, promotion, and the ‘pairing’ of stressed and unstressed syllables. In this view, the ending of Keats's line illustrates a permissible variation in English iambic verse, whereby the occurrence of two stressed syllables together can be compensated (in certain positions) by the pairing of two unstressed syllables. In Greek and Latin quantitative verse, some kinds of substitution are governed by the principle of ‘equivalence’ whereby one long syllable is equal to two short syllables, so that under certain conditions a spondee, for example, can stand in for a dactyl.