In a pipe organ, a flue pipe is any pipe that is sounded by a fipple, similar to that in a whistle or a flute a bec, rather than by a beating reed, see reed pipe. Most of the pipes in a typical pipe organ are flue pipes; Smaller organs may have only flue pipes. This article is part of the Pipe Organ Refactor Project. ... A fipple is a mouthpiece that uses a narrow windway and a blade-like edge to channel and vibrate air blown into it. ... A whistle is a one-note woodwind instrument which produces sound from a stream of forced air. ... A reed is a thin strip of material which vibrates to make music. ...

Most flue pipes are of one of three tone families:

• Flutes have the purest waveforms.
• Diapasons or principals have the strongest sounds, and are midway between flutes and strings in tone.
• String stops have the richest harmonics, and the pipes of these stops tend to be narrower to produce this effect.

Ranks of all three tone families may be either stopped or open, and made either of metal or of wood. Metal pipes are normally round in cross section, while wooden pipes are most often square. Originally this was for ease of manufacture, more recently it is also for reasons of tradition. The word diapason is used in a number of musical contexts. ...

Unfortunately, the above terms are far from standardised. Some builders, for example, use the term flute to mean a stopped flute or diapason, and/or the term diapason to mean an open diapason.

The most common flue stop not of any of these families is the rhorflot.