Harmony the Chords of the Major Scale

The last one being diminished ( often symbolized by vii dim or vii O ). The I, IV,V, and vi have special names. They are tonic, subordinant, dominant, and relative minor respectively. So in C-Major the relative minor is A-Minor and based on the sixth degree of the major scale.

These relationships apply to any key major scale.

These relations are very common in most music you will ever hear. There are many many songs from all eras and genres that use only the three chords I, IV, and V. The rest of the history of harmony involves additions, sometimes based on altering intervals in these chords, and modifications related to them.

It is extremely useful to know how these chords sound by ear.