The octave key on a saxophone is a key mechanism that allows the player to change octaves while playing. When the octave key is pressed, it opens or closes a vent located on the neck of the saxophone, which changes the length of the air column inside the instrument. This alteration in the air column length causes the pitch to jump up or down by one octave, allowing the saxophonist to play notes in a higher or lower octave without changing fingerings.
In other words, when the saxophonist plays a note with the octave key engaged, the resulting sound will be one octave higher than the fingerings would normally produce without the octave key. This allows the saxophonist to access a wider range of notes and play melodies or solos with a broader tonal palette. The use of the octave key is a fundamental technique in saxophone playing and is commonly employed in various styles of music, from classical to jazz to popular music.