How chorale fantasias are constructed?

Matthias writes that he's coming from the "Schnitger-Land" near Hamburg, Germany and his dream is to play Johann Adam Reincken's "An Wasserflüssen Babylon" in St. Katharinen, Hamburg which was Rencken's church at the time. For him it is very hard to find out the musical themes and cantus firmus (chorale melodies) in this many-sided masterpiece. In order to play such an awesome masterpiece, he wants to know all about the stylistic devices of the North German Baroque school and melodic lines in such organ works.

I'm sure it's not easy for Matthias to find out the compositional devises in North German chorale fantasias. They may be extremely elaborate in these multi-faced works. To make this task easier for him and for any of my subscribers who are interested in such intricate music, you can do the following things.

Get familiar with the chorale tune of the chorale fantasia. This might mean searching for the harmonized version of the chorale or a simple one voice melody. Note how many phrases this chorale tune has, what kind of key or mode, what kind of cadences you see at the end of each phrase.

Know that each phrase of the typical chorale fantasia of Buxtehude, Reincken, Weckman, Tunder, Bruhns, Scheidemann, and other North German composers might have any or all of the following techniques:

1. Ornamented chorale in the soprano or the tenor in 3 or 4 voices
2. Cantus firmus chorale in any of the parts in 2, 3, or 4 voices
3. Combination of cantus firmus and ornamentation techniques in 4 voices (the bass or the tenor has long chorale notes but the soprano or the tenor has unrelated ornamented melody (counterpoint).
4. Chorale ricercare with imitative counterpoint (fugal chorale)
5. Melodic echos over the cantus firmus in augmentation in the bass
6. Chordal echos for manuals only
7. Echo passages in two parts in sixteenth-notes

The composer might treat each of the above techniques at the tonic, dominant or subdominant level making it a fairly lengthy piece.

In the chorale fantasias of more conservative composers the themes or chorale melodies and their fragments are always clearly visible in the score. But with Reincken it's different.

He might start out with the cantus firmus melody in long notes but later develop the same chorale phrase into something more advanced when you don't easily see the notes of the chorale anymore. Most of them are there, though but you have to look deeper.

Sometimes you will discover the chorale notes hidden in the diminutions of the chorale where they can be found on the main beats of the measure. But often it's even more complex and creative than that.

Some notes are skipped, some played at different octaves, and some fall not on the beats but somewhere in between. And of course Reincken doesn't hesitate to use the entire range of the keyboard for his ornamentations. That's what makes his masterpiece so unique and remarkable.

Sight-reading:
Duo IV "Ave maris stella" (in Versets of 2, 3 and 4 voices, Fabordones, Intermedios, p. 4) by Antonio de Cabezón (1510-1566), a blind Spanish Renaissance composer and organist.

Hymn playing:
Faith of Our Fathers