Chords built on the degrees of a scale a group of pitches arranged from high to low covering one octave and using only notes of the scale are known as scale-tone chords. Here are the scale-tone triads of the C Major scale:
There are Major chords on I, IV, and V, minor chords on the ii, iii, and vi, and vii is a diminished chord a group of three or more tones forming a harmony .
Here are the scale-tone seventh chords of the C Major scale:
The I and IV have Major 7th chords. The ii, iii, and vi have minor 7th chords. The V has a dominant 7th and the vii has a half diminished, that is m7b5.
see also: Roman Numeral Analysis, Chord Construction, Scales and Modes