Other forms: part songs
A part song is a piece of music for three or more voices and no instruments. You'll most likely hear a part song sung by a choir or a vocal ensemble.
If you’ve heard a barbershop quartet sing, you’ve heard a kind of part song, where the voices sing different parts in harmony and without instruments. Part songs are rooted in religious contexts, but just like barbershop quartet songs, they can often be secular. The absence of instruments highlights the flexibility and harmony of human voices, which typically sing four parts — soprano, alto, tenor, and bass — though sometimes there are only three. Usually the melody is sung by the same voice part all the way through, with harmony provided by the other voice parts that is often quite complex and...beautiful!