The left thumb operates nine keys: B1, B1, C2, D2, D5, C5 (also B4), two keys when combined create A4, and the whisper key. The whisper key should be held down for notes between and including F2 and G3 and certain other notes; it can be omitted, but the pitch will destabilise. Additional notes can be created with the left thumb keys; the D2 and bottom key above the whisper key on the tenor joint (C key) together create both C3 and C4. The same bottom tenor-joint key is also used, with additional fingering, to create E5 and F5. D5 and C5 together create C5. When the two keys on the tenor joint to create A4 are used with slightly altered fingering on the boot joint, B4 is created. The whisper key may also be used at certain points throughout the instrument's high register, along with other fingerings, to alter sound quality as desired.