The following guidelines are taken from the Preface to the 6 Organ Sonatas, Op. 65 by Felix Mendelssohn which is written by the composer himself.
FF (Fortissimo) – Full Organ. With this indication you can use most of the stops on your instrument, including principals, flutes, and chorus reeds of various pitch levels. Add couplers if you want a bigger sound. However when in doubt, omit certain stops or couplers because there is the danger for the organ to sound too loud. In other words, listen how the instrument sounds in the room and make appropriate adjustments.
pp (Pianissimo) – Soft 8’ stop alone. Such register might be a soft sounding flute or a string stop. If you have many such stops available on your organ, try to find the one which has an original character.
F(Forte) – Great organ without some of the loudest stops. In this case you should probably omit the loudest reeds, such as Trumpets of 16’ and 8’ pitch level. A full principal chorus based on the 16’ (if available) with flutes of different pitch levels will do.
p (Piano) – Several soft 8’ stops combined. Two or more flutes and strings will usually sound nice. In some cases you can also use manual couplers.
Registration in the pedals. Use 16’ and 8’ stops together in the pedal, except where expressly stated otherwise. The composer specifically refers to the variation part of the 6th Organ Sonata, where you can find indication that a chorale tune (cantus firmus) “Vater unser im Himmelreich” should be played on the 8’ stop (perhaps the reed).
2 manuals – different tone color, without too great contrast. When registering pieces for two different keyboards, aim to use stops which are not too different in terms of volume level. Instead, choose the ones which produce a different sound character.
Suitable for other German Romantic music. It is interesting to note that Mendelssohn’s suggestions are valid not only for his music, but also for the works of other German Romantic Composers. This is based on the fact that the organs which were built at that time in the German lands share many similar concepts.
Follow these guidelines when registering organ music by German Romantic composers in general or pieces by Felix Mendelssohn in particular. As always, when adapting original registration indications to modern organs, have in mind the ideal sound that you want to produce and make necessary changes. If in doubt – choose the solution which is the simplest.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.
Or if you want to learn to improvise in the style of Bach? If so, I suggest you check out my 9 day mini course in Keyboard Prelude Improvisation.