Diatonic Triads in Major
Building a triad on every degree of the major scale, and naming each with Roman numerals.
Stack a third and a fifth above every note of the major scale, using only notes from that scale, and you get seven triads — one per degree. Because the scale's whole/half-step pattern is fixed, the quality of each triad is fixed too, in the same order every time: I major, ii minor, iii minor, IV major, V major, vi minor, vii° diminished.
Roman numerals encode both the scale degree (the number) and the chord quality (the case and symbol): uppercase for major (I, IV, V), lowercase for minor (ii, iii, vi), lowercase with a ° for diminished (vii°). This lets you describe a chord progression independent of key — "I-IV-V-I" means the same shape whether you're in C major or F major.
Three of these triads carry most of the weight: I is the tonic (home), IV and V are the subdominant and dominant — the two chords that create movement away from and back to the tonic. The rest (ii, iii, vi, vii°) color and extend that basic motion; you'll meet their specific roles in the functional harmony lesson.
In the exercise below, a triad from the C major scale lights up — identify which degree (Roman numeral) it's built on by ear and by its position on the keyboard.