If principals are the most important tonal family in the organ, then the principal chorus should be the single most important tonal effect within the instrument. How did this concept of a chorus originate?
The classical Greek mathematician Pythagoras was the first to analyze harmony and the integral ratios of the lengths of vibrating strings to the creation of octaves, thirds, etc. It was not until the Middle Ages that the true nature of these relationships to organ pipes became evident. Without the aid of stop actions, builders had to combine pipes into pitch relationships which had a pleasing or harmonious sound, and the integral ratios of pipe lengths (1/2, 1/3, 1/4, etc.) indicated a connection to the "natural" philosophy of the ancients. Equipped with these simple and compelling mathematical details, it was easy for the builders to extrapolate pipes of longer and shorter lengths and to create a chorus from different pitches. Thus the Blockwerk (ancient chorus of principals) was born, with its many pitches playing from a single key; some would have pipes at 16', 8', 4', 22/3', 2', 11/3' and 1'. When this uncontrolled chorus was broken up with the invention of stop actions, the larger low-pitched pipes came out as independent ranks, but the many small high pitched pipes remained as unbroken segments of the Blockwerk, called mixtures.
Unfortunately, stop actions also made it possible to eliminate parts of this ancient principal chorus, and after hundreds of years, most of the upper pitches disappeared. By the early part of the 20th century there was no chorus left; a single 8' diapason was the lonely reminder of a past glory. The revival of the principal chorus within our lifetime has been the signal tonal event for the modern pipe organ.
Planning a principal chorus is akin to designing a skyscraper; it must have height, grace, excitement and grandeur. A simple tall box lacks appeal, but with the right detail and style, the whole can become satisfying and useful. A helpful concept is that the principal chorus extends both up and down; the further up you go in pitch, the further down you must also go or risk top-heaviness. Likewise, a chorus which is all bottom is not a skyscraper.
It is not necessary for every division of a pipe organ to have a principal chorus, but it certainly helps. The Great is the home of the largest and most majestic principal chorus. It should start at 16' or 8' pitch and include other principal ranks at 4', 22/3', 2', 13/5' and then the mixture(s). A mixture is an assembly of small principal pipes usually pitched at octaves and fifths, though there are some examples with thirds. The normal Great mixture(s) might have from three to ten ranks of principals at 2', 11/3', 1', 2/3', 1/2' etc., an alternation of octaves and fifths.
The function of a mixture is to add brilliance and sparkle to the organ; it must blend with the tonal character of the lower pitched principals which undergird it. After setting down definite rules, let me say that most organ rules can be broken and that each situation calls for thoughtful and sensible design to establish the right sound.
Principals should be used in other divisions of the organ to secure the special tonal needs which are required. On the Positive, they give a lighter singing sound to echo the Great. In the Swell, they can supply an accompanimental sound which has definition and precise pitch. In the Pedal, principals give a strength and precision to the bass line which is essential. Few organs have the benefit of a 32' principal or even a 16', and some are without a mixture for the pedal, but it is hoped that the lack of principals which characterized organs early in this century has now changed and their presence is both appreciated and encouraged in all divisions.
The character of principal sound varies as needed in the organ, and this is achieved by using differing scales, wind pressures, voicing techniques, etc. When pipes of the same length (same pitch) but differing diameters are compared, it is found that the larger pipes give a fuller, flutier sound. Small diameter pipes have a stringy sound with reduced power. With proper design and voicing, it is possible to produce a satisfying principal sound in a small practice room or house. For large spaces it is necessary to have more sound, requiring a different design and voicing style.
The family of principals (Diapason or Montre, Prestant or Octave, Twelfth, Fifteenth, Mixture) is large both in tonal character and pitch variety. No organ is large enough to include every type of principal ever made, but even small organs should have as many as possible to give richness and strength to the sound. Sometimes a name may be misleading (the Stopped Diapason is not a Diapason in the real sense), but the success of principals in an organ is not the result of the names on the drawknobs, but the careful design and artistic voicing of the pipes. A beautiful principal chorus is a perfect example of the adage that the whole is greater than the sum of its parts; it is also true that individual members of the chorus may have great beauty all by themselves or combined with other stops.
I expect there will be a continuous development and refining of the use of principals in the organ, an indication that the instrument is not static, but always changing.