Why is so much English poetry written in iambic pentameter?

Scholars usually explain its success by comparing it with the other big English metre, four-beat verse: pentameter keeps a steady beat but leaves room for the rhythm of real speech. Poetic metre is measured in small groups of syllables called feet. An iamb is a foot made of an unstressed syllable followed by a stressed one, as in a-BOVE, and pentameter means five feet to a line. So a standard line is five iambs in a row: da DUM da DUM da DUM da DUM da DUM. Shakespeare's Sonnet 12 opens with a straightforward example: "When I do count the clock that tells the time."

This is the most common metre in English poetry. Chaucer first brought it into English in the 14th century, on the basis of French and Italian models, and one estimate from 1971 is that at least three-quarters of all English poetry since Chaucer is written in it.

Why did it win? Compare it with four-beat verse, the metre of nursery rhymes, skipping games, folk songs, ballads and marching chants. The scholar Derek Attridge points out that because pentameter has an odd number of beats, it does not impose itself on the natural rhythm of speech, and it frees the voice from the repeated tune of four-beat. Note the careful claim: it is not that pentameter is "natural" to English, as people often say. It is that the metre lets the varied pace and intonation of real speech be heard on top of the regular beat.

Poets also bend the pattern constantly. Shakespeare's line "Now is the winter of our discontent" opens with an inversion, a flipped first foot that reverses the order of the syllables.