The lyrics are the words that make up the song, and are usually composed of verses and choirs. The writer of the poems is a lyric poet. However, words for expanded musical composition such as opera are commonly known as "libretto" and its writer, as "librettist". The meaning of the lyrics can be either explicit or implicit. Some song lyrics are abstract, almost incomprehensible, and in such cases their interpretation emphasizes form, expression, scales, and symmetry of expression. A rapper can also create lyrics (often with a variety of rhythmic words) that are meant to be spoken rhythmically rather than singing.