Quatrain
A four-line stanza — Western poetry's most widespread stanzaic unit.
The quatrain is a four-line stanza, the Western tradition's most widespread unit: ample enough to develop an image or idea, brief enough to stay whole in memory. Its four lines naturally host the three main rhyme arrangements — couplets (AABB), alternate (ABAB), enclosed (ABBA) — each with its own dynamic: alternate moves forward, enclosed encloses.
In the sonnet, quatrains form the opening movement, the one that sets. On its own, the quatrain is also a complete form — that of the epigram, the versified maxim, the nursery rhyme: four lines are enough for a punch.
Example
A Shakespearean sonnet stacks three quatrains before the couplet strikes.