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.
Read more about this topic: Register Key