A foot is the basis of meter: that is, the regular unit of rhythm which, when repeated, makes up a verse. Although the basis of meter in the classical languages was "quantitative" — i.e., "long" and "short" syllables were based on the actual amount of time it took to speak the syllables — and some English poets made experiments in this direction, virtually all English feet are based on a pattern of stressed and unstressed syllables. Still, the terms are usually imported from Greek and Latin versification, and you may hear "long" and "short" where "stressed" and "unstressed" are meant.

Each common foot comprises two or three syllables: either one or two stressed syllables, and zero, one, or two unstressed syllables. The common feet in English:

Meter is usually described by giving both the kind of feet (above) and the number in each verse. The basic meters are: Although in most kinds of English verse one type of foot predominates in each line, substitutions are possible.

From the Guide to Literary Terms by Jack Lynch.
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Note: This guide is still in the early stages of development.
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