20.3The most important three chords first

In a major key, start by testing:

I, IV, V

In C:

C, F, G/G7

Many simple melodies can be harmonized largely with these.

So when listening to a simple song, don't initially ask about Dm, Em, Am, secondary dominants, borrowed chords, etc.

First see whether:

C / F / G7

explains the skeleton.