Poetic meter has two basic types: qualitative and quantitative. Quantitative meter is governed by syllable length and was commonly used in classical languages, such as Latin and ancient Greek. Qualitative meter is governed by syllable stress and is the de facto meter of English and the Romance languages. Although several English-speaking poets have experimented with quantitative meters, nearly all poetry in English is instead written using qualitative meters.
Metrical poems are divided into lines, with each line being composed of one or more feet. A foot is a particular combination of either long and short or stressed and unstressed syllables. The most common feet in English poetry are the iamb (unstressed-stressed, as in the word “concur”) and its opposite, the trochee (stressed-unstressed, as in the word “conquer”). Poems written using iambs or trochees are called iambic or trochaic, respectively, followed by the Greek prefix for the number of feet present in each line (mono-, di-, tri-, and so forth) and the suffix -meter. Thus, the most famous English meter is called iambic pentameter, because it consists of five iambs per line.
Meter in the Western literary tradition begins with Homer’s “Odyssey” and “Iliad”; the Greek tragedies of Aeschylus, Sophocles and Euripides; and the lyric poetry of Sappho and Pindar. The Latin poets, most famously Vergil, Horace and Catullus, adapted their meters from the Greeks, and Romantic Europe inherited them after Rome’s decline. In the 14th century, Geoffrey Chaucer popularized iambic pentameter in English through his adaptations of such Italian poets as Giovanni Boccaccio, while Thomas Wyatt and Henry Howard reignited interest during the 16th century through their translations of Petrarch. These translations paved the way for pentameter's adoption by Shakespeare, Milton and others.
Poems almost never slavishly adhere to a particular line or foot. Hamlet’s “To be or not to be--that is the question,” for example, has 11 syllables and is not perfectly iambic, despite being written in iambic pentameter. These two common variations on the basic meter are called extrametricality (the addition of an unstressed syllable at the end of a line) and inversion (the transposition of stressed and unstressed syllables following a caesura, or pause). In other words, the meter permits “that is,” a trochee, in place of an iamb because of the preceding dash, which creates a pause in the line.
Not all poetry is metrical, and, in fact, English has a long history of unmetered poetry, or “free” verse. Free verse, however, didn’t become truly popular until the 20th century, when such modernists as T. S. Eliot and Ezra Pound began to experiment radically with poetic structures and rhythms. These experiments inspired later generations, including the Beat poets, to cast off meter altogether, resulting in such seminal poems as Allen Ginsberg’s “Howl."