Re: 32\',64\'?
Hi Gareth,
No, stops aren't rests.
Imagine the simplest organ possible: one manual(=keyboard), and one pipe for each key. Usually, an organ manual starts at the c two octaves below middle c (and goes up to between two and three octaves above middle c, so the compass is between four and five octaves).
The lowest c has a frequency of approx. 65Hz (assuming a=440Hz), so the laws of physics tell you that its pipe must have a height of approx. 8 feet. If you align all the pipes, they get of course smaller and smaller as you go up the scale, like a pan flute. This array of pipes is called a
stop (well, I really should say "rank", but forget about that for the moment), and one then speaks of an 8' stop, referring to the height of its largest pipe.
Now organs usually have not one, but
several stops (i.e. several pipes for each key). The pipes of different stops may differ in shape (influencing the colour of the sound) or in size (changing the pitch), or both. For example, if the pipes of the previous 8' stop are aligned in front of you, one pipe for each key, you may imagine a second array of pipes behind the first, with each pipe having half the length (and thus sounding an octave higher) of that in front of it. So you now have an 8' stop and a 4' stop.
Organs then have a mechanism allowing the player to let the wind flow into each stop separately, via drawknobs next to the manual, one knob for each stop: pull it and the pipes of the corresponding stop will sound as soon as you press the keys; push it back in and the pipes will remain silent.
Press the middle c key and hold it down: you don't hear anything as long as you haven't pulled any stops. Now pull the 8' stop: you'll hear a middle c. Push the 8' stop back and pull the 4' stop: you'll hear the c one octave above middle c. Pull both stops, and you'll hear both.
In this example, you should not, however, think of the 4' stop as giving the means to play parallel octaves in a Liszt-like manner: its purpose is rather to
artificially add harmonics to the 8' stop. You may likewise have an 2'2/3 stop, a 2' stop, a 1'3/5 stop, etc. (adding 3rd, 4th, 5th harmonics, respectively: 2 2/3=8/3, 2=8/4, 1 3/5=8/5). You can then use these stop in any combination you see fit (of course, the 8' stop, sounding at the normal pitch, should always be drawn, except maybe for very special effects).
As I said before, you may also have several stops at the same pitch, with pipes of different shapes. The main parameter is the ratio diameter/length: if the diameter increases, the sound becomres softer, more flute-like. The material may change: the "main" stops have round metal pipes, but some stops have square wooden pipes. Another variation is that some stops have pipes that are closed at the top. There are also reed stops, where the sound is not produced by air vibrating in the pipe (like in a flute), but by a reed (like in a clarinet, or like your lips when playing the
), so that's an entirely different family of stops altogether.
Most organs also have
mixtures (in another post, you asked about those, too). A mixture is
one stop having
several pipes for each key, each pipe giving a harmonic for the corresponding pipe of the 8' stop. For example, a mixture may be a combination of a 2'2/3, a 2', and a 1'1/3 stop (giving 3rd, 4th and 6th harmonics). Each of these three individual arrays of pipes is then called a
rank of the mixture, and one talks about a
3-rank mixture.
So why have only one drawknob for all the ranks of a mixture at once, and not one knob for each rank individually, to add more possible combinations? Well, most of the time, the harmonics given by a mixture will change as you go up the scale. For example, a four-rank mixture may give the 6th, 8th, 12th, and 16th harmonics at the lowest c, but at some "break" point replace the 16th harmonic by a 4th harmonic, then replace the 12th harmonic by a 3rd harmonic at a second "break" point, etc. Thus, a mixture is really more complicated than just a bunch of stops aligned in parallel.
You also asked about "great". Well, as you probably have noticed, most (larger) organs have not one, but
several manuals. Each manual then has its own collection of stops. Usually, one manual is the "main" manual, called
Great in English,
Grand-Orgue in French, and
Hauptwerk in German.