The alto saxophone is a member of the saxophone family of woodwind instruments invented by the Belgian instrument designer Adolphe Sax. The alto, with the tenor, is the most common size of saxophone. It is larger than the soprano, smaller than the tenor, and is the size most used in classical compositions.

The alto saxophone is a transposing instrument and reads the treble clef in the key of E♭. A written C for the alto sounds as the concert E♭ a major sixth lower.

The range of the alto saxophone is from concert D♭3 (the D? below middle C - see Scientific pitch notation) to concert A♭5 (or A5 on altos with a high F♭ key). As with most types of saxophones, the standard written range is B♭3 to F6 (or F♭6).[1] Above that, the altissimo register begins at F♭ and extends upwards. The saxophone's altissimo register is more difficult to control than that of other woodwinds and is usually only expected from advanced players.