Let‘s assume you have written a piece which has a simple ternary ABA form. ABA form means that there are two short musical ideas and the first idea is repeated at the end as a capitulation. If one idea is about eight measures long, then your entire piece might be 24 measures long.
If you want to expand it into a more substantial organ piece, you can do a simple trick which is to transpose your entire composition to a new key. Most likely this key will be closely related to the home key. One of the most common transpositions you see is in the dominant key which is built on the 5th scale degree. So you only have to transpose your ABA piece a perfect fifth higher.
Sometimes you will run into problems of voice range while transposing. If this is the case, I would recommend you can switch soprano and tenor parts with one another. If your piece has only three voices (for example, the right hand, the left hand and the pedal part), then you can switch the right hand with the left hand part.
As you can see, your entire second section might be just a transposition of your first section but in a different key. In order to complete the piece, you can add a recapitulation of your original piece in a home key. So in reality, this also will be a piece in a ternary ABA form but on larger scale.
Apply my tips with your existing shorter organ compositions and you will see how they work very well in expanding your organ pieces.
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