1. Organ Technique. In this field, we learn the basics of organ playing. These may include depression and release of keys, legato touch, finger independence, glissando playing, finger substitution etc. The same holds true to the pedal playing. This area of study involves playing specially designed exercises which develop specific organ technique.
Some people do not consider the basics of organ technique very important and they start teaching organ with organ repertoire right from the beginning. I think that although this can be done, playing repertoire from the start might be sometimes too difficult for the beginner. However, the repertoire can be incorporated and mixed with the exercises quite successfully.
2. Organ Repertoire. This area, of course, involves practicing and performing many organ compositions from various national schools and historical periods. Because the organ repertoire is so vast (the earliest surviving music was composed about 700 years ago), every organist can learn something from it that is interested and useful.
Naturally, the Bach organ works are the main emphasis of most organist's repertoire, but various other important schools need to be learned as well. For example, very popular among organists today are French Classical, French and German Romantic, North German Baroque, 20th century French repertoire. However, I think one can benefit from playing other lesser played composers from the Italian and Spanish Renaissance, English Baroque composers etc.
3. Organ Registration. Organists would not have sufficient skills, if they did not know the basics of organ registration. This involves knowing how to register specific types of organ composition, how to mix the stops, how to achieve balance between manuals and pedals, how to adjust historical registration practices to different modern organs etc.
It is important for an organist to know how to make a good seamless crescendo, how to apply principles of terrace dynamic, how to make good use of the swell pedal etc. All these things can be learned if we study organ registration practices.
4. Sight-Reading. Organists also need to sight-read new repertoire pieces every day. This is the skill which will be very useful when organists have to accompany choirs or collaborate with other musicians. Without practicing sight-reading regularly, organists will have difficulty of reading and learning new music.
It often happens that we are given new music to perform with choirs, ensembles, and soloists just a few days in advance (or less). If a person does not have good sight-reading skills, he or she might have much trouble and stress learning to play these new pieces fast.
So you can see, how these four areas of study (organ technique, repertoire, registration, and sight-reading) are crucial, if the organist wants to be successful.
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