The alto flute is a flute that plays a fourth (half an octave) lower than the regular flute. It is longer than the regular flute. Sometimes alto flutes have a bent part at the top end.
The alto flute is a transposing instrument. This makes it easy for someone who plays the flute to play on the alto flute. The fingering for the note C on the flute will give a G on the alto flute.
The alto flute is not very often used, and only in very large orchestras. Maurice Ravel wrote a beautiful tune for alto flute in Daphnis et Chloé. Igor Stravinsky used one in The Rite of Spring. In The Planets by Gustav Holst there is a passage with three flutes and alto flute playing 4 part harmony.
The alto flute must not be confused with a bass flute which is even bigger and lower.
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Media related to Alto flutes at Wikimedia Commons