One way to spin out another melodic line that can be used as its own theme, or as a cantus firmus for counterpoint, is the use of augmentation.
Augmentation is the act of multiplying in ratio to the original line, the rhythmic values of the melody. So, a quarter-note would become a half-note, and a dotted eighth-note would become a dotted quarter-note.
Take a look at the example below:
Here at mm. 3-4, there is an augmentation and octave transposition of the first measure. Least to say, possibilities are endless!
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