Constrained writing is a literary technique in which the writer is bound by some condition that forbids certain things or imposes a pattern.
In poetry, formal constraints abound in both mainstream and experimental work.
Familiar elements of poetry like rhyme and meter are often applied as constraints.
Well-established verse forms like the sonnet, sestina, villanelle, limerick, and haiku are variously constrained by meter, rhyme, repetition, length, and other characteristics.
Outside of established traditions, particularly in the avant-garde, writers have produced a variety of work under more severe constraints; this is often what the term "constrained writing" is specifically applied to.