Constrained writing

Constrained writing is a literary technique in which the writer is bound by some condition that forbids certain things or imposes a pattern.
Constraints are very common in poetry, which often requires the writer to use a particular verse form.
Description.
Constraints on writing are common and can serve a variety of purposes. For example, a text may place restrictions on its vocabulary, e.g. Basic English, copula-free text, defining vocabulary for dictionaries, and other limited vocabularies for teaching English as a second language or to children.
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. For example:
The Oulipo group is a gathering of writers who use such techniques. The Outrapo group uses theatrical constraints.
There are a number of constrained writing forms that are restricted by length, including: