2.13This is the connection between scales and chords

This is a crucial mental shift.

A key is not just a scale.

A key gives you a family of chords.

In C major:

C Dm Em F G Am B°

These chords are closely related because every note comes from the same seven-note collection.

That is why they tend to sound naturally compatible.

So when you are trying to harmonize a melody in C major, your first candidate chords come from this family.

Not because you're forbidden from using anything else—but because these are the native chords of the key.