The iamb dominates English poetry for a simple phonetic reason: the language leans on unstressed function words (the, of, and, a) flowing into stressed content words. When that alternation locks into a repeating ˘ - pattern, poets call each pair an iamb, and the adjective is iambic.
Counting iambs names the line: one iamb is a monometer, two dimeter, three trimeter, four tetrameter, and five pentameter. Five iambs - iambic pentameter - is the workhorse of Shakespeare's plays and sonnets, Milton's epics, and most sonnet traditions. Three iambs (iambic trimeter) carries most hymn meters and ballad stanzas.
Poets rarely sustain pure iambs. The most common variation is the initial trochaic inversion (- ˘ opening the line), used deliberately by Shakespeare ('Now is the winter') to grab attention. A line with occasional inversions is still described as iambic if the underlying pulse holds.
To check your own lines, paste them into our Poetry Meter Checker: it maps every syllable's stress, divides feet with vertical bars, and reports whether the dominant pattern is iambic - including confidence percentages on substitutions rather than treating them as errors.