The choir is a group of singers, who sing sometimes in unison, sometimes in 4-part harmony. Generally, the highest harmony part is soprano (when sung by women or girls) or treble (when sung by boys). The next part is alto, still written on the treble clef. The next part is tenor, sung by men and written on the bass clef in hymn books, but written on the treble clef at one octave above true pitch for most other choral music. The deepest part is the bass, which is always written in the bass clef.