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Adding the Seventh
Builds on Triad Construction as Stacked Intervals
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A seventh chord takes a triad and stacks one more 3rd on top, adding a fourth note a 7th above the root. That extra note gives seventh chords their richer, more 'suspended in motion' sound compared to plain triads.
The most common seventh-chord qualities: major 7th (major triad + a major 7th above the root), dominant 7th (major triad + a minor 7th — the workhorse of blues and functional cadences), minor 7th (minor triad + a minor 7th), diminished 7th (diminished triad + a diminished 7th, splitting the octave into four equal minor 3rds), and half-diminished 7th, written m7♭5 (a diminished triad + a minor 7th — one semitone 'kinder' than a fully diminished 7th).
⚠️ Common mistake
Confusing diminished 7th and half-diminished 7th (m7♭5) — they share the same diminished triad base, but differ in that 7th: fully diminished uses a diminished 7th (very dark, symmetrical), half-diminished uses a minor 7th (slightly less tense).
📝 Summary
A seventh chord = a triad + one more stacked 3rd. Common qualities: major7, dominant7, minor7, diminished7, half-diminished7 (m7♭5) — each defined by its triad plus the exact size of that added 7th.
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