A pipe organ is a musical instrument that makes sound by vibrating columns of air inside pipes.
There are two ways a pipe organ makes sound: by beating the air with a reed (reed pipes) or by blowing air past a tongue (flue pipes). The former way is also used by saxophones, clarinets, and oboes. The latter way is used by whistles, recorders, and flageolets.
A pipe can only speak one note. The pitch of the note is determined by the length of the vibrating column of air within the pipe (length of the resonators in reed pipes and distance from the top of the mouth to the end of the pipe in flue pipes). A pipe speaks (makes sound) when air is blown into it.
To play the organ, you use a keyboard. All the controls of the organ are located on the console – the bench where the organist sits to play the organ. The console contains one or more manuals (keyboards you play with your hands, like a piano) and usually a pedal (a keyboard you play with your feet). A full-size manual has 61 keys (5 octaves plus one). A full-size pedal has 32 keys.
If a manual has 61 keys and a pedal has 32 keys, then why do organs have more than 93 pipes? Because pipes can be made in all different shapes and proportions. The shape and size of the mouth, width of the pipe, and material used to construct the pipe all affect the sound. So does the shape of the pipe – they are by default cylindrical but can also be conical, fluted, stopped, or partially stopped. A group of pipes that all have the same tonal characteristics and construction is called a rank. So, for example, the organ I’m building will have 6 different pipes that could speak when I pressed the lowest key on a manual. How do I specify which pipes I want to speak when I press a key?
In addition to the manuals and pedal, the organ has other controls. Most of these are stops. Stops can be in the form of draw knobs (that pull out of the console), rocker tablets (that operate like inverted light switches), or tongues:
A stop may select an entire rank, part of a rank, or sometimes pipes from different ranks. (The latter case is called a mixture – where more than one pipe speaks for each key depressed).
Stops are labeled with the name of the stop and the approximate length (in feet) of the longest pipe in the stop. Well, okay, not really. That’s only true for reed pipes and open flue pipes. Some flue pipes are stopped, meaning that they have caps or plugs in the end. This has the effect of doubling the length of the resonating column of air and making the pipe speak one octave lower. So if you select a stop of stopped pipes (like a Gedeckt 8′), the longest pipe in the stop will only measure about 4′ in length but will speak at the same pitch as an open pipe of 8′.
Ranks of pipes are grouped into divisions. A division usually has a keyboard dedicated to it, either the pedal or a manual. Some organs have more divisions than keyboards. The divisions that don’t have their own keyboards are called “floating” and must be coupled to a keyboard to be used.
Couplers connect a division to a keyboard. It need not be a floating division, either. A very common coupler is “Swell to Great.” This will connect the Swell division to the Great manual. It means that when you depress a key on the Great manual, any selected stops in the Swell division will also speak in addition to any selected stops in the Great manual. The Swell division also remains playable from the original Swell manual.
Things I didn’t cover here: Pistons, enclosures, suboctave and octave couplers, expression, and action. Those are coming soon.