2.12Let's prove it in G major

G major scale:

G A B C D E F♯

Build every triad:

G B D = G

A C E = Am

B D F♯ = Bm

C E G = C

D F♯ A = D

E G B = Em

F♯ A C = F♯°

Therefore:

G Am Bm C D Em F♯°

Again:

I ii iii IV V vi vii°

Same architecture.