A pipe organ is a wind machine with a very large switchboard. Keys pick the note, stops pick the pipes, and a pipe sounds only where the two agree. Start the blower, draw a few stops, and play.
Every stop knob below is one rank: a row of pipes, one per key, all with the same tone colour. The number on the knob is its footage, which says what octave or harmonic the rank sounds at.
Each column is a key and its pallet, the valve a key opens. Each row is a rank and its slider, the strip a stop knob slides open. Pipes are drawn to scale for their rank, longest on the left. A pipe speaks only where its key's pallet and its rank's slider are both open, and only if there is wind.
The sound of one key, split into its harmonics. Each drawn stop adds energy at its own place: a 4' stop lands on the 2nd harmonic of the 8' pitch, a 2 2/3' on the 3rd, a 1 3/5' on the 5th. Stacking stops builds a tone out of harmonics, the way additive synthesis does.
An open pipe sounds at a wavelength about twice its length. The lowest C on a standard organ keyboard is about 65 Hz, and half its wavelength in air is a little over eight feet. So a rank that plays at ordinary written pitch is called an 8' stop, after the length of its longest pipe.
Halve the length and the pitch goes up an octave: a 4' stop sounds an octave above the key, 2' two octaves above, 16' an octave below. Fractional footages are the in-between harmonics: 2 2/3' sounds an octave and a fifth up (the 3rd harmonic), 1 3/5' two octaves and a third up (the 5th).
A stopped pipe, capped at the top, sounds an octave lower than an open pipe of the same length. That is how a 16' stopped rank fits in about eight feet of pipe, and why stopped flutes have a hollow tone: a capped pipe mostly produces odd harmonics.
Sounds at , from a pipe about long.
Uses a speed of sound of 343 m/s and ignores end correction and pipe width, which make real pipes a little shorter than this. The low C here is C2, the bottom key of a 61-note manual.
How the key reaches its pallet is the organ's action. In a mechanical or tracker action, thin wooden or metal rods connect key to pallet directly, so the player feels the valve open. In electro-pneumatic and electric actions, the key closes a switch and a magnet or a small bellows opens the valve, so the console can sit far from the pipes.
Most pipes in an organ. Air from the foot is shaped into a sheet that strikes a sharp lip at the mouth, the same way a recorder or whistle works. No moving parts. Width, shape, and whether the top is open or stopped give the families: principals (the organ's core sound), flutes (rounder, wider), and strings (narrow and keen).
A brass tongue beats against an opening, like a clarinet's reed, and a resonator on top shapes the tone. Reeds such as the trumpet are bright, loud, and full of harmonics. On many consoles their knobs are engraved in red, as the trumpet is here.
Bigger organs have several keyboards (manuals) and a pedalboard. Each controls its own division, a separate set of ranks with its own chest: the Great is the main chorus, the Swell sits in a box with shutters for getting louder and softer, and the Pedal holds the deep 16' and 32' ranks. Couplers let one keyboard play another division's stops too.
A mixture is one knob for several high ranks at once, here three, sounding octaves and fifths far above the key. It gives a full chorus its sparkle. On a real organ, a mixture's pitches drop back an octave at a few points up the keyboard (it "breaks") so the top notes do not climb out of hearing; this toy leaves that out.
Good explanations already exist; this page adds a small console you can play while watching the wind chest and the harmonics. Worth a look:
The sounds here are simple synthesized tones, chosen to make the harmonics easy to see, not to imitate any real organ.