When harmonizing the major scale by stacking thirds, a chord on each degree of the scale is created.
This produces the diatonic triads, a natural sequence of major, minor and diminished chords.
The pattern is consistent in every major key:

These chords are the backbone of tonal harmony.
They give you ready-made tools to write progressions,
find substitutes and understand modulation.
The I chord feels like “home” or the gravity center.
The IV and V create movement and resolution.
The VI acts as a smooth detour to minor moods.
The II and III help with voice leading and soft transitions.
The VII adds instability and tension, a call for return.
