Quatrain

In the description of verse forms, a quatrain is a group of four lines considered as a unit, or sub-unit, of a poem. The four lines may be of any length: four pentameters make up quatrains in sonnets, but the quatrain in the traditional ballad stanza is a group in which the first and third lines are of four iambic feet, and the second and fourth of three. In this case, each stanza is a single quatrain, where the sonnet typically has two or three quatrains. Common rhyme schemes in quatrains are a b a b and a b c b.

(The plausible a a b b is usually best seen as two couplets.)

Etymological note: The word quatrain, which came into English from French in the 16th century, derives ultimately from the Latin quattuor, ‘four’.