Originally, the length of the air column in organ pipes that determined a note’s pitch. For example, the pipe that produces Middle C on a pipe organ is approximately two feet long; the stop that opens a pipe in this octave is called a “2′ stop”. Doubling the air column length lowers the pitch by one octave, so a 4′ stop produces a C one octave below Middle C; an 8′ stop is two octaves lower, etc. On the largest pipe organs the lowest pipes are in the 32′ range (four octaves below Middle C), and are assigned to the bass pedals. Conversely, halving the stop length raises the pitch one octave, creating 1′, 1/2′, and even smaller lengths.
Not all stops are doubles or halves of each other. To produce the harmonic “G” above an 8′ “C”, for example, a 5-1/3′ stop is used. Its octaves are then reflected in 10-2/3′, 2-2/3′, 1-1/3′, and other multiples or divisions. A pipe organ has at least one pipe to produce every pitch on its manuals and pedal board, and usually many more to produce different timbres such as reeds, brass, and voices.
The term “stop” soon became used to describe the control on an organ console that selects a particular timbre (called a “register” or “registration”) and organists began somewhat standardizing favorite combinations of settings, which they circulated among themselves. Organ builders began incorporating these combinations into their instruments, creating the first “presets” as we know them today.