A category of literary composition determined by literary technique, tone, content, or even (as in the case of fiction) length. The distinctions among genres and categories are flexible and loosely defined, often with subgroups. The most general genres in literature are (in loose chronological order) epic, tragedy, comedy, and creative nonfiction. They can all be in the form of prose or poetry. Additionally, a genre such as satire, allegory or pastoral might appear in any of the above, not only as a sub-genre, but as a mixture of genres. Finally, they are defined by the general cultural movement of the historical period in which they were composed. Genre should not be confused with age categories, by which literature may be classified as either adult, young adult, or children's. They also must not be confused with format, such as graphic novel or picture book.