I have recently begun successfully extemporising complete fugues. It is as cool as I thought it would be.
That said, some of you, I am sure, are doing this at a much higher level, and can help with my questions:
1) Do you map out the tonal centers of your fugue, and they keys of the expositions, before you begin, or let it happen as it happens?
2) What considerations do you undertake in crafting the subject to ensure that the stretto will be effective?
3) The rules of counterpoint are fantastic. Are you able to keep conscious tab on them throughout the improvisation, or do they simply inform, through your exposure to properly written counterpoint, your perception of whether the fugue sounds correctly or incorrectly led?
4) Do you ever reintroduce a voice that has been removed from the texture other than by stating the subject?
5) Can you invert or retrograde your subject without prior preparation? Any tips for struggling brains?
6) Anything else to impart to the novice fugue extemporiser?
I should like to study how to do this further. I can't offer more than admiration for those who can do this -- I get too lost in the tonality and can't keep track of the fugue subject without writing it down.
I think of it roughly like this: Subject - whether hymn tune or variant, or something else, I think in voices - like three voices, and plan which voice the subject will appear first. Having stated the subject in the first voice, then there has to be a bridge passage to lead into the second statement. The passage does not have to modulate into this key (typically the dominant of course) but to suggest the V. This is done by inflecting toward that area with the introduction of an accidental. (For ex., if I am in G Maj for the first statement, I will introduce a C# to imply the V. Conversely, after the second voice comes in (remember you've planned which voice this will appear) you simply 'uninflect' the C# to C natural in the next passage to imply a return to the tonic, where you have planned the third voice to enter. Of course the subject may enter the voices in any order, and it is cool to see what the subject may imply - like simply a rising subject can suggest a gradual arch, by appearing in the lowest voice first, then the second and then the highest etc.
When you have completed the exposition, interludes may be on another manual, or reduced registration, or just two voices. Imitation is very good and trying to create other figures that may be a variant of the subject. During this, the aim it to modulate to another key area, often the IV. It works well (only one way to do things) when you reach the new key, you go back to the stronger manual or registration to state the subject again. This rough idea of the interludes can be greatly varied to as much as you are able to. At the close of any number of these interlude-exposition cycles, you will want to lead toward the tonic. Many clichés work well here, such as a long pedal point on V (we are playing the organ after all) which leads to a grand statement of the subject in the tonic. If you are accomplished, this is a place to try augmentation, stretto ect. These techniques are difficult and have to be practiced quite a bit...
How well one can do this is like anything it depends on how much you practice. It's a good idea to spend some time each practice period with one aspect - such as stating the subject and the bridge into the next key etc. As well, writing out contrapuntal exercises found in any textbook is very helpful, as well as harmonizing chorales. Just my .02
For finding subjects, I noodle around during practice times and try things out...simple is often better (and easier to remember). If a subject is particularly good, I will write it down so that I can remember it. There are some subjects that are so versatile they can be used repeatedly and reveal different things (as Bach shewed in The Art of the Fugue).
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