The pipe organ is the largest and most complex of all musical instruments. It produces sound by driving pressurized air through rows of pipes, which are selected via keyboards (manuals) and a pedalboard for the feet. A large organ can have thousands of pipes, ranging from the size of a pencil to over 30 feet tall. Musicians use 'stops'—knobs that control which sets of pipes receive air—to change the timbre, allowing the organ to mimic flutes, trumpets, or even human voices. Because it can produce a massive volume and sustain notes indefinitely, it was the only instrument capable of filling the vast acoustics of cathedrals before the invention of electronic amplification. The largest functional pipe organ in the world is the Wanamaker Organ in Philadelphia, which has 28,750 pipes.