The Major Triad Chords

A major triad chord is three notes: a root, the note four semitones above it, and the note seven semitones above it. C E G is the major triad on C, and every other one copies those two distances from a different starting note.

Triads are where music theory stops being abstract. Build a triad on each note of a major scale, using only notes from that scale, and the qualities always come out in the same order: I, ii, iii, IV, V, vi, vii°. In C major those seven are C, Dm, Em, F, G, Am and B°.

So a major key contains three major triads, on degrees I, IV and V. The chord on vii comes out diminished, and the other three come out minor. Change key and the letters change; the order never does.

The formula for major triad chords is:

I-ii-iii-IV-V-vi-vii°.

That means every single major scale is comprised of those seven chords. If you're not familiar with this notation, you can read up on roman numeral analysis.