How Chords Are Built From Scales

The short answer: take a scale, pick any note, stack every-other-note on top of it (1-3-5 counting from that note), and you've built that degree's chord. Do it on all seven degrees of a major scale and the chord qualities come out major, minor, minor, major, major, minor, diminished — always, in every major key. Nobody chose that pattern; the scale's uneven spacing forces it.

The every-other-note machine

C major: C D E F G A B. Build on C by skipping: C, skip D, E, skip F, G → C-E-G, a C major triad. Build on D: D-F-A → D minor. On G: G-B-D → G major.

Same machine, same scale — different chord qualities out. Why?

Why the 2 chord comes out minor

The qualities are decided by the gaps. C up to E spans 4 half steps — a major third, so C's chord is major. D up to F spans only 3 — minor third, minor chord. The culprit is the half step hiding between E and F inside D's stack: it shrinks the interval.

The major scale's half steps sit between degrees 3-4 and 7-1, and their positions relative to each stack are what stamp each chord:

Memorize the pattern as a rhythm: Ma-mi-mi-Ma-Ma-mi-dim. It's the same in every major key forever, which is why it's worth owning cold: know the key, and you know all seven chords without computing anything.

Suddenly, songs make sense

This table is the skeleton key for "why these chords":

• G-C-D-Em songs — I, IV, V, vi of G. The four strongest chords of any key; thousands of songs use only these (the axis progression story).
• Why Am shows up in C major songs — it's not a visitor from A minor; it's C's own vi chord.
• Why an A major chord in the key of C sounds like an event — it's not in the table. Out-of-family chords are spice precisely because the family is so predictable. (Chords outside the key.)
• Why the 7th degree barely gets used — diminished chords are unstable; pop mostly skips the vii° or dresses it up.

Keep stacking: seventh chords

The machine doesn't stop at three notes. Stack one more every-other-note and each degree gets a four-note chord: Imaj7, ii m7, iii m7, IVmaj7, V7, vi m7, vii m7♭5. That V7 — the only dominant seventh in the family — is special enough to power half of Western harmony, and the full four-note story lives in seventh chords explained.

On the fretboard

Theory-to-neck translation: in any key, the I, IV, and V roots sit in one compact cluster on the E and A strings (in G: frets 3, 8... or 3 and its neighbors via the octave lattice), and the harmonized scale is playable as a ladder of triads or barre-chord fragments up one string set. Play G-Am-Bm-C-D-Em up the neck as triads on strings 2-3-4 once, and this whole article becomes muscle memory.