Rock operas, concept albums, song cycles and oratorios all differ from a conventional rock album, which usually includes songs that are unrelated to each other in terms of storyline. The rock opera style sometimes overlaps with concept albums, song cycles and oratorios. More recent developments include metal opera and rap opera (sometimes also called hip-hopera). The category a particular work falls into is, to some extent, defined by the intent and self-definition of the work by its creator, as long as the creator's interpretation does not stray too far from the accepted definition of what constitutes a rock opera. The formal distinction may be that the rock opera tells a coherent (if sometimes sketchy) story, often with first-person lyrics sung by characters; while a concept album or song cycle sets a mood or maintains a theme. Some albums share characteristics of more than one category. Tommy, one of the best known rock operas, also had a rock musical production. On a technical note, it is often inaccurately stated that the phrase "rock opera" is, in terms of both music and theater, a misnomer. The term is only a misnomer when it is incorrectly applied to a work that has been miscategorized. Opera consists of individual singers acting out a specific character within a drama. The same is also true of a legitimate rock opera. If a work is comprised of singers who sing a story, but do not act it out, it is not a rock opera, and should not in the traditional sense, be categorized as such.