If we build a chord starting on C and use the tones from the C major scale, we use C, E, and G. If instead we use the C minor scale, the first, third, and fifth notes are C, E-flat, and G. This difference in the third degree, between E and E-flat, turns the chord itself from a major chord into a minor chord. All of us, even without musical training, can tell the difference between these two even if we don’t have the terminology to name them; we hear the major chord as sounding happy and the minor chord as sounding sad, or reflective, or even exotic.
