Major and Minor Triads

The major triad and the minor triad are the two chords Western music leans on most. Built on the same root, they differ by exactly one note - the 3rd - and that single semitone is the whole difference between bright and dark. G major is G, B, D; G minor is the same G and D with the third dropped to Bb.

Both chords come up constantly in all three of their positions - see Triad Inversions for how a triad carries its identity even as the bass note underneath it changes.