Stringed instrument that's often as tall as its seated musician

CELLO

This deep-voiced orchestra staple sits between the player’s knees and is tuned an octave below a viola, often anchoring the harmony in classical quartets.

This clue describes a large stringed instrument played while seated, where the player’s height often matches the instrument’s size. The answer is "cello," short for violoncello, which is known for its deep, rich tone and towering frame when positioned upright. The hint plays on the visual of a musician sitting with the cello’s body resting on the floor, making it appear nearly as tall as them. No wordplay here—just a straightforward description of the instrument’s physical presence.