As a side point, just because the key sig has no sharps or flats does not mean the song doesn't change key. The key sig only dictates what the notes are NOT the chords, also tunes can have no sharps or flats in the key sig and be written using accidentals, eg song in G only has 1 sharp (F#). could use the key sig for C and use notate the F# using '#'accidental.

The final chord of a song is normally the home Key, also don't just restrict your chord selection to just 1 4 5 6. ALL the chords are open to use and also chords from other keys. Music that stays in one key can get boring not always but it can.


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