Principals. This is the main stop of the organ. Usually we see the principals in the organ façade, they are visible front pipes. In the large instruments, most of the time the principals are built from metal. However, some organs have wooden principals as well. The façade pipes are mostly built from tin. Inside principals have a large percentage of lead. Although there are a large variety of principals in various organ building traditions, generally the sound of the principal stops is firm and clear. In countries like Spain and Italy principals also had a vocal singing tone quality.
The principals are very well suited to accompany congregational singing in hymn playing. It is very common to add several principals of different pitch levels (16’, 8’, 4’, 2’, etc. and mixtures) to form Organo Pleno registration or the Principal Chorus. Such use of principals is a standard way to register Baroque compositions which are not based on pre-existing chorale melody (preludes, fugues, fantasias, toccatas, ciacconas, passacaglias etc.). Many of the chorale-based works can be played with single principals or various combinations of principal stops, as well.
Flutes. These stops imitate various instruments of the flute family (recorder, orchestral flute, piccolo etc.). Like principals, they can be built both from metal and wood. The diameter of these pipes is usually wider than that of the principals. Therefore they produce a warmer, rounder sound. Flutes are built of various shapes: open, stopped, chimney flutes, conical, over blowing etc. The shape of the pipe makes a big difference on the sound.
Flutes are very well suited for the performance of lyrical organ music. However, pieces of playful character can be effectively played using various flute combinations, even gapped registration, such as 8’ and 2’.Like principals, flutes can be used without the foundation of 8’. Solo flutes of 4’ or even 2’ sound very playful. There is a saying among organists that even the worst organ has at least one interesting stop. Usually it is a 4’ flute.
Strings. The string stops imitate string instruments, such as violin, viola, cello, double-bass, or even viola da gamba which was a very popular instrument in the Baroque period. Strings have much narrower diameter than that of a principal. Therefore their sound is soft, narrow, and gentle. The strings are more commonly built out of wood but there might also be metal stops as well.
The strings are especially important for the performance of the Romantic slow, meditative, and gentle organ music, such as Adagio, Priere etc. If the organ has several strings stops, such as viola, salicional, gamba and others, all of them work very well together. It is also very common to use the celeste sound together with strings. Because the celeste stops are tuned slightly sharper (sometimes slightly flat as well) they produce an undulating sound in combination with 8’ strings. This sound is similar to the effect of the tremulant.
Reeds. This is a very special stop family. They are very different from principals, flutes, and strings in their construction and sound production. They have brass plate called a tongue which vibrates when the air goes into the pipe. The sound is strengthened by the wooden or metal resonators which are placed on top of the pipes.
Reeds can be of two kinds: solo reeds and chorus reeds. All of them imitate various kinds of wind instruments, like oboe, clarinet, French horn, krummhorn, dulzian, bassoon, trumpet, trombone, tuba and many others. The solo reeds can be used as a solo stop on a separate manual or division. They are especially well suited for the performance of organ chorales, and other chorale-based compositions. The chorus reeds are generally louder. If they blend well with other stops, we can use them to strengthen the sound of the principal chorus both in the pedals and in the manuals.
NOTE: This is by no means a comprehensive treatise about the organ stops and their registration. This article is of course just a very brief and generalized overview of organ stop families to help you get started with organ registration. If you would like to know more about organ registration, I highly recommend The Registration of Baroque Organ Music by Barbara Owen. As Journal of the American Musical Instrument Society writes, "In this book, Barbara Owen has created a rich resource of historical information coupled with strategies for interpreting that information on today's instruments." Sixteenth Century Journal also adds that "... Barbara Owen has succeeded admirably in distilling three centuries of organ registration practice into a volume less than three hundred pages long.... Anyone with an interest in the history of the organ and its music... will not want to ignore this book." I personally use this book as a guide for most of my organ recitals.
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