Open flutes are nearly always present at 8ft. and 4ft. pitches and under a diversity of shapes and names.—clarabella, wald flute, tibia, melodic, flute ouverte, etc. Viols, however, are thought to mar the sovereignty of diapason tone, and so are less common on the English Great Organ than abroad.
The usual complement of Great Organ reeds amounts to trum pets, i6ft., 8ft., and 4ft., going by the name of contra (tromba) or bombarde Oft., tromba, trumpet or posaune, 8ft.; clarion or octave (tromba) 4ft. For a small Great Organ a single tromba or trumpet 8ft. suffices, or else the reed class may be omitted alto gether from this section. To which it may be added that most builders to-day prefer to furnish all their chorus reeds with har monic or double-length trebles which stand in better tune than the non-harmonic.
Swell Organ.—The Swell Organ (Fr. Recit; Ger. Schwell werk) again almost explains itself. For inasmuch as organ pipes are naturally incapable of crescendo and diminuendo, a whole section of stops is played from a separate manual, and made expressive by artificial means. That is to say, all the pipes of the Swell Organ are enclosed in a large box which is faced on one side with a set of balanced shutters, not unlike the louvres of a venetian blind. As the shutters are opened electrically by the organist from the console, the volume of tone increases ; as he closes them, it diminishes. Most people can easily recognise the sound of a full swell by the peculiar "tingling" effect it has when the shutters are nearly or completely closed. This effect indeed gives the best indication as to the basis of the Swell Organ : for it is shown to depend almost entirely on reeds and mixtures. Were all other stops omitted, a "skeleton" Swell would suffice for this effect, consisting of double trumpet i6ft., trumpet 8ft. (cor nopean), clarion 4ft., plus brilliant mixtures to crown the whole.
At the same time a Swell Organ confined to a reed basis would not be practical politics; and so it comes about that other stops, both reed and flue, are required also here. The oboe, for instance (=echo horn), and the unda maris or voix celeste (a pulsating stop made up of two out-of-tune dulcianas) are almost universal.
Hardly less so are narrow scale gedeckts of some kind (lieblich or rohr) in the foundation pitches. Nor is the entry of the diapason series delayed, though its stops will be of lighter calibre than those on the Great. The Swell mixtures belong properly to the diapason series, and only by design to the reed chorus, and so should not be voiced to emulate the sound of smashing glass, as sometimes happens. Then salicional (echo vd), spitz flute (dg), aeoline (echo v), wald flute (f ), etc., should all be added to the list of unobtrusives, but usefuls.
In a small Swell Organ a contra-oboe or fagotto Oft. generally appears as its first double ; for a reed has the advantage over, say, a bourdon or contra-salicional i6ft. in taking up less room inside the swell-box, while contributing directly to the reed foundation.
Choir Organ.—The Choir Organ (Fr. Positif ; Ger. Oberwerk or Positiv) belongs to one or other of two well defined types. It is either an Echo Great Organ in the true sense and in the old style; or else it stands in a swell-box and is less exclusive in caste.
If a miniature Great Organ, it comprises a lightly blown diapason series (i6ft. to mixtures), a gedeckt or two, and perhaps even a low-pressure trumpet. In its other capacity it has an ensemble of mild flutes and viols, rather than one of pure diapason complexion, though of course there is no actual ban on diapasons. Viola da gamba, viola d'amore, fugara (v), chimney flute, cor de nuit, quintaten (g), flauto amabile, wald flute, har monic flute (f) at 8ft., 4ft. and 2ft. pitches, the gemshorn family at all pitches—none of these are out of place so long as the key-note is gentleness and moderation in scales and wind pressures. If the Swell Organ contains a voix celeste of two salicionals (echo dv), occasion is sometimes taken to provide the Choir with a contrasting flute celeste of two gedeckts. The dul ciana (echo d), or salicional 8ft., is of course indispensable and in larger instruments it may appear at i6ft., 4ft. and/or 2ft. also. Nor does a delicate mixture of dulciana or salicional pipes ever come amiss, though it is surprising how few organ-builders care about stops of so refined a tone.