Marcel Dupre, the master French organist had ornamentation rules which he applied for Baroque organ music (especially for the music of Bach). He had rules, when to play upper note trills and main note trills, when to stop the trills, how to accelerate etc.
His rules are very good but there are many instances when they are not valid, even in Bach.
Trills in the Baroque music are much more varied than he understood in his day. The is so much variety in various national styles, so if you play Italian, Spanish, English, Portuguese, French, Dutch, North German, South German, Central German etc. - every national school had its own rules which obviously Dupre was not referring to when he wrote his set of rules.
Bottom line: we need to be very flexible these days and do lots of research in terms of fingering, ornaments, registration, and other elements of performance practice.
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.
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Organists of Vilnius University , creators of Secrets of Organ Playing.
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