Have you ever noticed that when we ask a question, our voices tend to wander higher in pitch? Even as you were reading that pervious question, the voice in your head must have had direction in their tone, contour, and phrasing. When someone answers a question, the voice goes in the opposite motion and lowers in pitch.
Sure, can you find examples where this does not happen? Yeah, but it is a phenomenon of a natural occurrence of contrary motion in contour.
Say you have a melodic fragment, and idea, that needs some kind of “answer” to it. First, observe the melodic intervals, direction, and shape of the melodic line.

Now, flip that shape upside-down in your head. For example: if it went up, have the melody go down now in roughly the same interval area. This adds a complemental structure to the melody.

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