Triad · Theory

Chord Major Triad Minor Triad Diminished Triad Augmented Triad Scales and Harmony Harmonic Function

A triad is the result of stacking two thirds: root, third and fifth — three notes.

All of its variation comes from one thing: how wide each of those two thirds is.

Lower third	Upper third	Result	Example (from C)	Character
major	minor	major triad	C–E–G	bright, stable
minor	major	minor triad	C–E♭–G	dark, inward
minor	minor	diminished triad	C–E♭–G♭	unstable, must resolve
major	major	augmented triad	C–E–G♯	suspended, unanchored

There are only four, and there can be no more — each third is either major or minor, so the combinations number 2 × 2 = 4.

Two observations

One: order decides quality. A major third plus a minor third gives entirely different chords depending on which comes first (major triad versus minor triad). The same logic as interval inversion: the up-down relation is structural information.

Two: the distribution is very uneven. Of the seven triads stacked on a major scale: three are major, three are minor, one is diminished, and none is augmented. The augmented triad never occurs in a diatonic scale, which is exactly why it sounds like it belongs to no key.

Triads and tonality

Triads are the smallest load-bearing unit of tonality: the tonic chord fixes the key's centre, the dominant chord fixes the pull back to it. That relation is covered under harmonic function, and which triads a given scale can produce under scales as harmony.

Diagram: four combinations

Listen: the four triads

In order: major, minor, diminished, augmented. The first two can stand; the last two must move.

Common misconceptions

"There are many kinds of triad." Only four, because each of the two thirds is either major or minor.
"Diminished and augmented triads are theoretical." Diminished triads appear constantly in keys (degree 7 in major, degree 2 in minor); augmented triads never appear in a diatonic scale but are common in chromatic writing.
"Rearranging the notes keeps the same chord." After inversion the root and the name stay the same, but the sound and function change (see inversion).
"A triad's notes must be adjacent." The voicing can be wide open, so long as the three degree names remain (see voicing).

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Listen in the library

Scales, Arpeggios, and CadencesC13:22Arpeggio exercises play triads one note at a time — major and minor alternate within one set of materialDetailsSound Ye the Trumpet CallC10:35An organ hymn in block chords throughout — the cleanest sample of triadsDetailsSonata in A Minor, K.175C13:50A sonata in A minor — minor and diminished triads used where they can be heard directlyDetails

All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).

Sources

三和弦由根音、三音、五音构成；四种性质由两个三度的宽窄组合决定，属和声学通则
四种三和弦的出现频率与稳定性差异，为通行和声学表述

Used for fact-checking only; all prose is original.

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