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A triad is constructed of two stacked thirds: a root, a third above it, and a fifth above the root. The quality of those thirds (major or minor) determines the chord quality (major, minor, diminished, augmented). Understanding triads as interval stacks clarifies how seventh chords and extensions are built.
You know that an interval is a measured distance between two pitches — a major third spans four half steps, a minor third spans three. You also know that a triad has three notes: a root, a third, and a fifth. What you're learning here is how to see those two ideas as one: a triad is nothing more than two stacked thirds. This perspective is more powerful than memorizing chord formulas, because it makes the entire system of chords transparent and extensible.
Start with a C major triad: C–E–G. From C to E is a major third (4 half steps). From E to G is a minor third (3 half steps). So a major triad = M3 on the bottom, m3 on top. Now build a D minor triad: D–F–A. From D to F is a minor third (3 half steps). From F to A is a major third (4 half steps). Minor triad = m3 on the bottom, M3 on top. The two qualities are the same two intervals, just in reverse order. This is why major and minor triads share an interval-content but have opposite "color" — the placement of the larger interval at the bottom versus the top changes the triad's character entirely.
The other two triad qualities follow the same logic. A diminished triad stacks two minor thirds: m3 + m3. C–E♭–G♭ is an example. Because both thirds are small, the fifth from root to top note is only 6 half steps — a diminished fifth (also called a tritone). This interval is unstable and tense, which is why diminished chords have a characteristic restless quality. An augmented triad does the opposite: M3 + M3. C–E–G♯ gives you a major third from C to E, another major third from E to G♯, and a resulting augmented fifth (8 half steps) from root to top. The symmetry makes augmented chords ambiguous — you can't easily tell which note is the "root" by ear.
The real payoff of the interval-stack view is that it makes seventh chords and beyond immediately comprehensible. A seventh chord is just a triad with another third stacked on top. A major seventh chord is M3 + m3 + M3 (three thirds stacked). A dominant seventh chord (the most important in tonal music) is M3 + m3 + m3. A minor seventh chord is m3 + M3 + m3. You don't need to memorize each formula separately — you just need to know what the stacked intervals are and listen for the quality at each step. From there, adding ninths, elevenths, and thirteenths (extended chords in jazz) follows the exact same logic, just adding more thirds above. The triad-as-interval-stack is not just one way to think about chords; it is the generative structure that makes all of tonal harmony coherent.
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