5.25Melody often tells you when harmony should change

Suppose a melody holds G for several beats.

You could keep C underneath.

But you might change:

C → Em → G7

while the melody remains G.

Why?

G belongs to all three chords:

C = C E G

Em = E G B

G7 = G B D F

Same melody note.

Changing harmony underneath changes its meaning.

This is fundamental to arranging.