A short prelude or interlude could have a form of a Binary Exposition which is about 16 measures long. It can be a piece which stands on its own or it can be a part of the larger structure.
Such exposition has the two main elements - a theme and a commentary. A theme is a musical phrase which lends itself for development. A commentary is a different phrase which contrasts somewhat with a theme.
So your Binary Exposition of 16-20 measures might look like this:
1) Theme of 4 measures.
2) Commentary ending on the dominant of 4 measures.
3) Repetition of the theme of 4 measures.
4) Commentary ending on the tonic note of 4 measures.
5) (Optional before 4) Commentary ending on a different note of 4 measures.
Remember, the melody does not have to be in the right hand part - it can easily be placed in the tenor voice and played on a different manual with solo registration. The pedal part is optional.
What to do with the accompaniment? Many things - you can do a simple four-part harmony, add a melodic and rhythmic figure etc. You are only limited by your imagination. If this is the case, look a the pieces of your favorite composers for models.
You can improvise your Binary Exposition on the organ, compose it on the paper or both (preferably). Give it a try today. It's easier than it sounds from this description. Creating your own music is one of the greatest joys musician can have.
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