Lyrics are words that make up a song, usually consisting of verses and choruses. The writer of lyrics is a lyricist. The words to an extended musical composition such as an opera are, however, usually known as a "libretto" and their writer, as a "librettist". The meaning of lyrics can either be explicit or implicit. Some lyrics are abstract, almost unintelligible, and, in such cases, their explication emphasizes form, articulation, meter, and symmetry of expression. Rappers can also create lyrics (often with a variation of rhyming words) that are meant to be spoken rhythmically rather than sung.