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.