Diatonic Harmony
The short version
Only using the chords that live inside your key.
Build a triad on each note of the major scale and you get seven chords that all belong: major, minor, minor, major, major, minor, diminished.
Why you'd careStay diatonic and everything fits automatically. Once you know the rule, breaking it becomes a deliberate choice.
Go deeper
Numbered with Roman numerals: I ii iii IV V vi vii°. Capitals are major, lowercase minor. Musicians talk in numerals so a progression can be moved into any key.
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