Chords in a key: how to build them from the scale

C major uses C, D, E, F, G, A, B. Take each note in turn and add the note two steps above it, then the note two steps above that — every other note of the scale.

Starting on C gives C, E, G — a C major chord. On D: D, F, A — D minor, because the third above D within this scale is F, a minor third. On E: E, G, B — E minor. On F: F, A, C — F major. On G: G, B, D — G major. On A: A, C, E — A minor. On B: B, D, F — B diminished, because both intervals came out small.

Nothing was chosen. The qualities fall out of the scale's own pattern of whole and half steps, which is why they are the same in every major key.

Written as Roman numerals, with capitals for major and lower case for minor: I, ii, iii, IV, V, vi, vii°. In C that is C, Dm, Em, F, G, Am, Bdim.

Numerals are worth learning because they are key-independent. A progression written I–V–vi–IV is C–G–Am–F in C major and G–D–Em–C in G major: same relationships, different notes, and the shape of the song survives the transposition.

The three major chords — I, IV, V — are the ones most folk and blues songs use exclusively. The three minor ones — ii, iii, vi — supply contrast, and vi is the relative minor, which is why a song can lean on it without leaving the key.

A natural minor key uses the same seven notes as its relative major, so it produces the same seven chords in a different order: i, ii°, III, iv, v, VI, VII. In A minor: Am, Bdim, C, Dm, Em, F, G.

In practice minor-key music usually raises the seventh degree to make the fifth chord major — E instead of Em in A minor. That single change gives the dominant its pull toward the tonic, which natural minor lacks, and it is why harmonic minor exists at all.

On guitar the fastest route is shapes rather than arithmetic: in the key of G the chords G, Am, Bm, C, D, Em fall under the hand in first position, and a capo moves all of them to another key at once while the shapes stay.

On piano the visual is better: play the scale with the right hand, then play every other note. The chords are literally the white keys in C, which is why beginners are taught that key first.

Either way, the useful test is by ear. Play I, then IV, then V, then I again. Once that sounds like home–away–tension–home, the numerals have stopped being labels and become sounds you can predict.

Knowing the seven chords of a key is what makes chord progressions understandable rather than arbitrary: most songs use four of them, and the popular ones recur because those relationships are strong.

It also works backwards. Given a set of chords in a song, matching them against the seven of each key usually identifies the key in seconds — which is exactly what a key finder does mechanically.

Beyond that lies borrowing: chords taken from the parallel key that do not belong to the seven. Those work precisely because the ear knows what should have been there.

Once you know which chords belong to a key, the next question is how a phrase using them ends — that is cadences. The same chords rearranged give inversions, and every key has a relative and parallel partner sharing most of its notes.