Alto is a singing voice that is lower than soprano but higher than tenor in many choral arrangements. In a choir, altos usually sing the harmony parts that support and enrich the melody the sopranos carry.

When composers write music for four vocal parts, they typically arrange them as soprano, alto, tenor, and bass. The alto part often requires a rich, warm tone that fills in the middle range of the harmony. Many famous singers sing alto parts, and the part suits people who can sing comfortably in that middle register without straining to reach the very highest notes.

The word also describes instruments that play in a similar range. An alto saxophone sits between the higher soprano sax and the lower tenor sax. An alto recorder plays lower than a soprano recorder but higher than a tenor recorder. In each case, alto indicates that middle range that adds depth and richness to musical ensembles.