Register key
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The register key is a key on a clarinet which is used to play in the second register; that is, it raises the pitch of most first-register notes by a twelfth (19 semitones) when pressed. It is positioned above the left thumb hole and is operated by the left thumb. The same key used in combination with the left first finger key which produces the written note A4 (in scientific pitch notation) is used to play B♭4. Some clarinets, particularly bass clarinets and lower, have separate keys, or a more complex key mechanism, to control two or three separate holes for playing B♭, for playing the lower notes of the second register, and for playing the upper notes of the second register. The analogous key on the saxophone and the oboe, the octave key, raises the pitch by an interval of an octave.
