Celestes are a significant variant of the string family of organ pipes. A celeste rank speaks at a very slightly different pitch to its unison rank, for example the Violin and Violin Celeste. The difference in pitch is extremely small; so small as not to be unmusical, but enough to give a very slight undulating effect (“celesting”), perhaps one or twice per second. This rate is dependent on the frequency difference between the unison and the celeste pipes, so for ‘A’ pipes where the unison is tuned to 440 cycles per second (Hertz) and its celeste partner is tuned (say) to 442 Hertz will when played together produce a celesting effect of 2 Hertz, or 2 beats per second.

The celesting rate is determined during tuning; in general for theatre organ work where the strings can be quite strident, a relatively fast rate can be acceptable in the mid and tenor compass. It should however be minimised or even completely absent, with the unison and celeste pipes tuned dead to each other in the treble, to avoid a rather un-musical harshness. To achieve this, each pipe on the celeste has to be tuned separately against the unison pipe rather than in octaves to itself. Consideration of the mathematics of pitch will show that if this is done then the celesting rate will increase dramatically as one goes up the compass.

The musical effect of celestes is to impart the impression of a multiple orchestral string ensemble to a string chorus. It is far more subtle than the effect of the tremulant; each has its place and the two should not be confused.

In construction, the celeste rank is, basically, identical to the unison rank, but may be of a very slightly smaller scale (maybe one or two notes narrower ) than the unison. It may also be regulated very slightly more softly.

At the console, celestes may be playable separately, with a separate stopkey for unison and celeste, or they may be combined onto one stopkey as a space-saver. In such cases there may be a separate control stopkey to silence the celeste ranks.

In the chamber, the celeste most usually has its own wind chest, but Comptons and Christies sometimes combined them on one Chest with a single, shared action; in this case, Christies provided Sliders to silence either rank for tuning purposes. In these cases the stopkey will be labelled in the plural, eg “Muted Strings” or “Strings II ranks” or similar.

The celeste effect can be applied to virtually any organ pipe; Whilst string celestes predominate, it can be highly effective with soft flues such as the flute or quintadena, or hybrid string/flute stops such as the highly regarded Skinner Erzahler. Its use in other areas is generally considered musically inappropriate