The pipe organ is generally the largest (usually by far), and most magnificent in the opinion of many people, of musical instruments. It produces its unique sound by forcing pressurized air through pipes when selected by pressing keys on one or more piano-like keyboards and usually a pedalboard as well. Each pipe produces a different combination of pitch, timbre and volume, and the number of pipes ranges from as few as a dozen for the smallest organs to more than 33,000 for the largest pipe organ. A continuous supply of pressurized air allows notes to be sustained for as long as the corresponding keys are pressed, unlike the piano and harpsichord, whose sound begins to dissipate immediately after a key is pressed. Mainly custom-built, no two pipe organs are identical, and they vary greatly according to the country and the era in which they were constructed.