NOTES FROM THE PIANO BENCH
Why can one melody note fit several chords?
A melody note does not tell you the entire harmony. E can be the root of Em, the third of C or the fifth of Am. Its role depends on the notes you put around it.
Treat the note as a shared point
Hold E on the piano and play C–G underneath. Change to A–C, then G–B. The top note stays still, but its surroundings tell three different stories. This is not a problem with one answer key; it is a choice you can learn to hear.
Context narrows the options
If the melody moves from E to a long F, an F chord may become a natural option. If the phrase is heading towards a C-major ending, G7–C can supply direction. You may also want to preserve a particular bass line. A convincing harmonisation considers several moments together.
Make a comparison you can hear
Use the same melody note, rhythm and loudness when comparing chords. Change one thing at a time. Describe the difference in your own words before collecting more terminology. Then try your alternatives in a whole phrase: an interesting moment is not always the best continuation.
What does this mean for improvising?
You need not choose between theory and your ears. Theory gives you a manageable set of candidates; your ears judge what suits your idea. When you find a version you like, move the whole pattern to another key. That teaches the relationship rather than just the keys under your fingers.