Iamb is a metrical foot consisting of an unstressed syllable followed by a stressed syllable: da-DUM. “To-DAY,” “a-LONE,” “de-STROY.” The iamb is the dominant foot in English-language poetry because it mirrors the natural rhythm of English speech, which tends to alternate unstressed and stressed syllables.
Iambic pentameter — five iambs per line — is the most important meter in English verse. Shakespeare’s plays, Milton’s Paradise Lost, Wordsworth’s The Prelude, and most of the English sonnet tradition are written in iambic pentameter. Its prevalence is not arbitrary: five iambs (ten syllables) approximate the length of a natural English sentence, making the line feel neither cramped nor sprawling.
In practice, iambic verse is never perfectly regular. Poets substitute other feet — trochees, spondees, pyrrhics — to avoid monotony and to create emphasis. A trochaic substitution at the start of an otherwise iambic line (“NÈVER, NÈVER, NÈVER, NÈVER, NÈVER”) creates a strong reversal that the ear notices. The tension between the expected iambic pattern and the actual rhythm is where much of formal verse’s energy lives.