Organ stops and pipes come in many different pitches, which reinforce different parts of the harmonic series and create a wide variety of tone colors.
The unison pitch, or the pitch that would be heard when playing the corresponding notes on the piano, is referred to as 8' or 8 foot. This is roughly the length of a unison-pitched open organ pipe producing the lowest C on the keyboard. Pipes sounding an octave higher are referred to as 4', and pipes two octaves higher 2'; these add brightness to the sound and are typically voiced softer than 8' pipes. Pipes sounding an octave lower are referred to as 16' and add gravity to the sound, but may cause it to become muddy if used incorrectly.
The pedals are pitched an octave lower than the manuals, so the main pitch is 16' and the octave above is 8'. Pedal 16' stops are usually doubled with 8' to make the low notes easier to discern. Larger organs may have 32' or even 64' pedal stops, which produce a characteristic rumbling sound that can be felt as well as heard.
There tends to be a greater variety of stops at 8' than any other pitch, though the extent of this differs depending on the style of organ. Baroque organs have many mutations and mixtures, while Romantic or symphonic organs have many 8' stops with different tone colors and less higher-pitched stops.
Mutations and mixtures
Stops referred to as mutations are pitched at non-octave intervals of the harmonic series and add a distinct timbre. The most common mutation stops are 22⁄3' (referred to as Nazard or Twelfth), pitched an octave and fifth above the unison; 13⁄5' (referred to as "Tierce"), pitched two octaves and a major third above unison; and 11⁄3' (Larigot), pitched two octaves and a fifth above unison.
Mixtures are multi-rank stops where multiple pipes are sounded for every note; the number of ranks is given with the stop name (e.g. "Mixture IV"). The pipes are pitched at consecutive octaves, fifths and sometimes major thirds, and serve to reinforce higher overtones and add brilliance to the sound. The relative pitches of most mixtures do not remain constant as one ascends from the low end to the high end of the keyboard; instead a high pitch is dropped out at some point and substituted with a lower pitch. Here is an example structure of a four-rank mixture:
C2 (low C) 1-1/3' - 1’ - 2/3’ - 1/2’ C3 2’ - 1-1/3' - 1’ - 2/3’ C4 (middle C) 2-2/3' - 2’ - 1-1/3' - 1’ C5 4’ - 2-2/3' - 2’ - 1-1/3' C6 5-1/3' - 4’ - 2-2/3' - 2’
A different type of mixture is the cornet, which consists of 8', 4', 22⁄3', 2' and 13⁄5' (some may lack the 8' or 4'). It has a nasal sound and is often used as a solo voice.
Some organs may also have mutations and mixtures that reinforce the 16' or 32' harmonic series instead of 8'; resultants are mutations in the pedal that attempt to create a 32' or 64' tone where installing real 32' pipes would be too costly.
Hammond organs follow the same system for pitches, but have drawbars instead of stops. Each drawbar is a slider that controls the volume of a particular pitch, and generates a flute-like tone with little overtones. Drawbar pitches found on most Hammond organs include 16', 8', 51⁄3', 4', 22⁄3', 2', 13⁄5', 11⁄3', and 1' for the manuals, and 16' and 8' for the pedals.