The pipe organ is a musical instrument that produces sound by driving pressurised air through the organ pipes selected from a keyboard. Because each pipe produces a single tone and pitch, the pipes are provided in sets called ranks, each of which has a common timbre, loudness (volume), and construction throughout the keyboard compass. In other words, a rank is a set of pipes all having the same tonal quality, but having a range of different pitches. Most organs have many ranks of pipes of differing pitch, timbre, and volume that the player can employ singly or in combination through the use of controls called stops.