Register key

The register key is a key on the clarinet that 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.