In music, an alto is a singer with a vocal range somewhere between a tenor and a soprano. A typical alto will have a range from around the F below middle C to the D a ninth above middle C. In four part choral harmony, the alto is the second highest voice. Altos originally sang from music written in the alto clef, but now use the treble clef.

Although both men and women may have voices in the alto range, the word is usually used to mean a female singer. However, choirs singing early music frequently include male altos, also called countertenors. In English church usage, the term alto is sometimes exclusively used to mean a boy with this range, while contralto is used for a female singer. However, this is not done consistently, and for most practical purposes, alto and contralto can be thought of as synonyms (the phrase "boy alto" can be used if there is a chance of misunderstanding).

The word alto is often applied to instruments to indicate their range in relation to other instruments of the same group. In common usage, particularly among jazz musicians, the word has become synonymous with the alto saxophone. In US usage the alto horn is an Eb saxhorn, a brass instrument.

Some well known contraltos include:

• Marian Anderson
• Janet Baker
• Kathleen Ferrier
• Ernestine Schumann-Heink
• Ewa Podleś

Derivation

In medieval polyphony the principal voice was the tenor. When additional voices were added, they were called contratenor, "against the tenor." When two such voices were added they were called contratenor alto and contratenor bassus, indicating high and low respectively. So alto essentially means high voice.