In the field of second language acquisition, there are many theories about the most effective way for language learners to acquire new language forms. One theory of language acquisition is the comprehensible output hypothesis.
Developed by Merrill Swain, the comprehensible output (CO) hypothesis states that learning takes place when learners encounter a gap in their linguistic knowledge of the second language (L2). By noticing this gap, learners become aware of it and may be able to modify their output so that they learn something new about the language. Although Swain does not claim that comprehensible output is solely responsible for all or even most language acquisition, she does claim that, under some conditions, CO facilitates second language learning in ways that differ from and enhance input due to the mental processes connected with the production of language. This hypothesis is closely related to the Noticing hypothesis. Swain defines three functions of output: