Examples of use
In major keys, major chords are found on the I, IV and V (1st, 4th and 5th) degrees of the scale.
In D major, that means D, G and A. These three chords form the basis of a huge number of popular songs.

In a minor key, a major chord is found on the III, V and VI (3rd, 5th and 6th) degrees of the scale.
For example, in G minor, there are major chords on Bb, Eb and D