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Triads (three-note chords) can be built on each scale degree using every other note of the scale. In major keys, the triads on I, IV, and V are major; on ii, iii, and vi are minor; and on vii° is diminished. In minor keys, the pattern differs based on which form of minor scale is used. Understanding this system reveals the harmonic foundation of a key.
Build all seven triads in a major key on staff paper. Play them on an instrument. Identify the quality of each (major, minor, diminished). Repeat for minor keys.
Students sometimes forget that triads use non-consecutive pitches (skipping every other scale degree). Another error: applying major-key triad qualities to minor keys without accounting for the different interval structure.
A triad is a three-note chord built by stacking intervals of a third — that is, by taking every other note of a scale. From your work with major and minor scale construction and interval quality counting, you have all the tools needed to build and identify every triad in a key. The key insight is that when you stack thirds using only the notes of a given scale, the resulting triads inherit the scale's interval structure, which means different scale degrees produce triads of different qualities.
Let's work through C major. On scale degree 1 (C), take C–E–G: C to E is a major third (4 semitones), E to G is a minor third (3 semitones). A major third stacked below a minor third = a major triad. On scale degree 2 (D), take D–F–A: D to F is a minor third (3 semitones), F to A is a major third (4 semitones). Minor third below + major third above = minor triad. On scale degree 7 (B), take B–D–F: B to D is a minor third, D to F is a minor third. Two stacked minor thirds = diminished triad. You don't need to memorize that "vii is diminished" — you can derive it from counting semitones. But after doing this enough times in enough keys, the pattern becomes automatic: I–ii–iii–IV–V–vi–vii° for major keys.
The minor key case is more complex because the natural minor scale produces a different interval pattern, yielding a different set of triad qualities: i–ii°–III–iv–v–VI–VII (in natural minor). The critical difference from major is the v chord: in natural minor, the chord on scale degree 5 is a *minor* triad rather than a major one. This matters because the dominant-to-tonic resolution that defines functional harmony depends on the leading tone — the note a half step below the tonic that creates strong upward pull. Natural minor's fifth scale degree does not contain the leading tone; it contains the subtonic (a whole step below the tonic) instead. When composers want a strong authentic cadence in a minor key, they typically raise the seventh scale degree to create a major V chord, which is why the harmonic minor scale exists.
The practical workflow for triad construction is: (1) identify the root (the scale degree you're building on), (2) take the scale note a third above (skip one scale note), (3) take the scale note a fifth above the root (skip another scale note). The resulting notes are all diatonic — no accidentals needed beyond what the key signature supplies. Count the semitones between root and third, and between third and fifth, to identify the quality. Over time, this process should become fast enough that building all seven triads in a key takes under a minute — the foundation for everything in harmony that follows.
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