Triad Construction: Major, Minor, and Diminished

Core Idea

Triads are three-note chords built by stacking thirds: a root, a third (either major or minor), and a fifth (either perfect or diminished). Major triads have a major third and perfect fifth; minor triads have a minor third and perfect fifth; diminished triads have a minor third and diminished fifth.

How It's Best Learned

Construct triads on a staff and keyboard starting from various roots. Play examples and listen to the distinct colors: major (bright), minor (dark), and diminished (tense).

Common Misconceptions

A triad is defined by its interval structure, not the order of its notes—C, E, G and G, C, E are the same C major triad in different inversions.

Explainer

From your prerequisite work with intervals and basic triads, you know that intervals are measured in semitones and have quality (major, minor, perfect, diminished, augmented). You also know that a triad is a three-note chord. Now these two ideas combine precisely: the quality of a triad is entirely determined by the intervals between its notes. There is no other ingredient. A major triad is not a C-major-triad because it starts on C — it is a major triad because of its internal interval structure. Change that structure by a single half step and you change the triad type.

Every triad can be described by two stacked thirds: the interval from the root to the third, and the interval from the third to the fifth. The major triad stacks a major third (four semitones) plus a minor third (three semitones), producing a perfect fifth (seven semitones) from root to fifth. C-E-G is a major triad: C to E is a major third; E to G is a minor third. The minor triad reverses the order: a minor third (three semitones) plus a major third (four semitones), still producing a perfect fifth. C-Eb-G is a minor triad: C to Eb is a minor third; Eb to G is a major third. The key insight is that major and minor triads both have a perfect fifth from root to fifth — what distinguishes them is only where the middle note sits. In a major triad the middle note leans toward the top; in a minor triad it leans toward the bottom.

The diminished triad is different in a fundamental way: it stacks a minor third plus a minor third (three plus three), producing a diminished fifth (six semitones) from root to fifth — an interval one semitone smaller than a perfect fifth. B-D-F is a diminished triad: B to D is a minor third; D to F is a minor third. The diminished fifth (also called a tritone) is an unstable interval that wants to resolve, which is why the diminished triad sounds tense and cannot serve comfortably as a resting point. In tonal music, the diminished triad appears most commonly on the seventh scale degree, where its instability drives motion toward the tonic.

The practical test for triad type is: identify the root, count up four semitones (major third) or three semitones (minor third) to find the third, then count to the fifth. But a faster method, once internalized, is to hear the emotional quality directly. Major triads have a stable, open, often cheerful character — the major third ring is bright. Minor triads are stable but darker, with the compressed minor third creating a more inward or solemn quality. Diminished triads are unstable and tense. These are not arbitrary associations; they follow directly from the physics of the overtone series and centuries of musical convention that trained Western listeners to hear them this way. Build each triad type from several different roots until the construction is automatic, then drill recognition by ear so the interval math becomes a description of something you already hear.