Register key
The register key is a key on woodwind instruments that is used to play in the higher registers.
Clarinet
On the clarinet, the register key is used to play in the second register; that is, it raises the pitch of most first-register notes by a twelfth 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 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 octave key is a key on the saxophone or oboe that raises the pitch of most notes by an octave when pressed, so that similar fingerings can be used for two different octaves.
Saxophone
On a saxophone, the octave key is positioned next to the left-hand thumb rest. Pressing the octave key opens the top tone hole in the neck of the saxophone. Alternatively, whenever the G key is fingered, the top tone hole closes and a small tone hole is opened near the top of the body.Some baritone saxophones, notably those made by Yamaha, also have three octave tone holes. The third one is used to prevent a rough transition from G# to A.