What's an “organ?”
If you go back a century or more, the word “organ” in the musical sense referred to a pipe organ that was located in a church or cathedral. Nowadays, a musical instrument referred to as an “organ” doesn’t necessarily produce its sounds using physical pipes. Rather it is a keyboard instrument, with more than one manual keyboard and a pedal keyboard, that produces sustained sounds by some means (often electronic) and has a system analogous to the stop system in pipe organs for altering the sounds in real time. Also, organs are now found in homes, concert halls, schools, bars and rock fests as well as in churches.
Organs designed for church use, whether pipe or “electronic,” usually maintain the concepts and terminology common to pipe organs. Such concepts are these:
Organs generally have more than one manual keyboard in addition to a pedal keyboard. These keyboards are called “divisions.” Manual keyboards have 61 keys (or notes), which span 5 octaves plus an extra top C note. A full AGO pedal keyboard spans 32 notes starting with C and ending on G. In a three-manual organ the divisions are usually called Great, Swell, Choir and Pedal, names that have pipe-organ significance.
Each division (keyboard) contains “ranks” of sounds that have pitches that cover the span of the keyboard. A rank on a pipe organ likely consists of 61 pipes that exhibit consistent sound quality over that pitch range of five octaves. Ranks are turned on or off by switches referred to as “stops.” The terms “ranks” and “stops” are often used interchangeably.
There are four families of stops (i.e., types of sounds) present in church organs: flutes, strings, diapasons, and reeds. Flute sounds have little harmonic content and therefore often sound pure, light, and/or mellow. Strings, on the other hand, are rich in harmonics but are usually fairly soft. Diapasons have that louder, churchy sound. Reeds are rich in harmonics and often rather loud. Reeds add important color to the ensemble sound. They can also be piercing or biting in nature.
In a pipe organ, flutes, strings and diapasons have much in common in that their tones are produced as a result of an air stream being directed across an opening in a pipe, much like the case of an orchestral flute. Such pipes are called flue pipes. The sound of a reed pipe is produced by a vibrating reed.
The word “pitch” has dual meaning in organs. In addition to referring to the pitch of individual notes, as in the above paragraph, the word “pitch” also refers to the overall pitch setting of a rank. A rank can have the expected pitch---middle C sounds like middle C---or it can speak an octave or more higher or lower. The overall pitch of a rank is designated by what would be the length of the longest pipe in that rank in a pipe organ. A rank that speaks at expected pitch is an 8’ (8-foot) rank. A rank that speaks an octave higher is a 4’ rank. Two octaves higher is a 2’ rank. An octave lower is a 16’ rank. Organs often have ranks spanning from 32’ to 1’. An occasional organ will have a 64’ rank, but most buildings can’t accommodate anything that massive. Also, pitches that low would be mostly inaudible in the lowest octave inasmuch as the lowest C would be speaking at 8 Hz.
Most church organs have some ranks of pipes that speak at pitches that are not octavely related to 8’ pitches but rather are harmonically related. An example would be a rank that speaks at the third harmonic of the 8’ rank. Such a rank would have a pitch designation of 2 2/3’. For the technically minded, the relationship between pitch, pipe length and the speed of sound is this: Wavelength multiplied by frequency (aka pitch) = speed of sound. The length of a pipe that is open on the end is one half of the wavelength of the sound wave produced in the pipe. Therefore, 2 x pipe length x frequency = speed of sound. In the example above of the 64’ stop, 2 x 8 x 64 = 1024 feet per second.
Stops are designed to be used singly or in combination with other stops. The various divisions are conceptually designed as separate organs and, therefore, each division will have stops designed to be used in ensemble along with one or more solo stops. The stops that are selected at any given time are referred to as “registration.”
Couplers allow divisions to be coupled to each other. A three-manual organ would allow, for example, the Swell and Choir manuals to be coupled to the Great. With these couplers selected, all stops on the Great, Swell and Choir divisions would be playable in ensemble from the Great manual. There are also, for example, 4’ couplers that allow manuals to couple to themselves and play notes an octave higher along with the unison pitch. A typical three-manual organ would also allow the coupling of each manual division to the Pedal division.
Organists often have need to change registrations on the fly. Therefore, most organs have the capability to instantly go from one registration to another with “preset” registrations waiting to be called up at the push of a button. Literally.
The organist determines registrations ahead of time and assigns them to numbered push buttons called “pistons.” Normally, there are pistons allocated to each division plus a separate group of pistons called “generals” that affect all divisions at once.