Seventh Chords

Stack one more third on top of a triad and you get a seventh chord — four notes instead of three, named for the interval between the root and the top note. The extra note adds color and, in most cases, extra instability that demands resolution.

The most important one by far is the dominant 7th (major triad + minor 7th, e.g. G-B-D-F): it contains both a tritone (B-F) and the leading tone, making it the single strongest engine of harmonic motion in tonal music — this is the chord that most wants to resolve to the tonic.

The other common types: major 7th (major triad + major 7th, lush and stable, common on I and IV), minor 7th (minor triad + minor 7th, warm and relaxed, common on ii and vi), half-diminished 7th (diminished triad + minor 7th, written ø7, common on vii° in major and ii° in minor), and diminished 7th (diminished triad + diminished 7th, symmetrical and maximally tense, common in minor keys).

You already know how to build these — a seventh chord is just "triad, plus one more third." Try building a few in your head from the triad qualities you learned in the last lesson before moving on.