From the Greek,'through all', the Diapason is the main chorus flue stop of the classical organ, and is found in almost all theatre organs as well.

Construction:
Open, metal*. Larger scale examples (eg Diaphonic Diapason) may have a leathered upper lip, and narrower scale examples (eg Horn Diapason) may have a bridge.

Tonality:
Broad, warm sound midway between flute and string. May voiced as a powerful, dull stop (esp. Diaphonic Diapason) or more lightly with more overtones (esp. Horn Diapason, Violin Diapason).

STOP NAMES

Diaphonic Diapason

Diaphonic Diapason with the 16' extension of diaphonic pipes. Usually a heavy, large-scale stop with leathered upper lips.

Open Diapason

Standard name for stop - confusingly, may also have a diaphonic 16' octave. Generally, less heavily voiced/of narrower scale than the Diaphonic Diapason.

Horn Diapason

Smaller scale stop, often but not exclusively used by Wurlitzer as a second Diapason on larger organs when the 16' octave will be diaphonic (metal) and labelled 'Bass' on the stopkeys. May also be termed Violin Diapason. Usually with a tuning slot and, in some cases, roller across the mouth.

Dulciana

A very quiet member of the Diapason family (often, incorrectly, considered a string stop). May be partnered with a celeste, Unda Maris in Wurlitzer nomenclature.

Octave

4' Diapason stop. Horn Diapason 4' may be labelled Octave Horn on Wurlitzers.

Fifteenth

2' Diapason stop. On many theatre organs is derived from a string rank rather than the Diapason itself, in order to give a more balanced ensemble.

* The Stopped Diapason, usually wooden, is closer to the flute family in tonality and rarely appears on the theatre organ.