Wikipedia
The alto clarinet is a woodwind instrument of the clarinet family. It is a transposing instrument pitched in the key of E, though instruments in F (and in the 19th century, E) have been made. In Europe it is sometimes called a tenor clarinet. In size it lies between the soprano clarinet and the bass clarinet, to which it bears a greater resemblance in that it typically has a straight body (made of Grenadilla or other wood, hard rubber, or plastic), but a curved neck and bell made of metal. All-metal alto clarinets also exist. In appearance it strongly resembles the basset horn, but usually differs in three respects: it is pitched a tone lower, it lacks an extended lower range, and it has a wider bore than many basset horns.
The range of the alto clarinet is from the concert G2 or G2 (in the second octave below middle C, bottom line of the bass clef) to E6 (in the second octave above middle C), with the exact upper end of the range depending on the skill of the player. Despite the broad range, the instrument is virtually always scored in the treble clef, but it is possible to play it in the alto, tenor, baritone, or bass clef.
Most modern alto clarinets, like other instruments in the clarinet family, have the Boehm system or Oehler system of keys and fingering, which means that this clarinet has virtually identical fingering to the others. The alto clarinet, however, usually has an extra key allowing it to play a low (written) E, and a half-hole key controlled by the left-hand index finger with a vent that may be uncovered to assist in playing the altissimo register.