Seventh Chords & Inversions

One more third

A triad is three notes stacked in thirds: root, third, fifth. Stack a fourth note — another third on top, seven scale steps above the root — and you have a seventh chord. That one extra note does something triads cannot: it adds tension, a note that wants to move. Jazz, blues, soul and most film music are built almost entirely from sevenths, and the single most important chord in classical harmony, the dominant seventh, is one too.

Because the triad underneath can be major, minor or diminished, and the added seventh can be major (11 semitones above the root) or minor (10), there are several flavours. Five cover nearly everything you will meet:

• Major seventh (maj7): a major triad plus a major seventh — C E G B. Soft, dreamy, slightly bittersweet; the sound of a sunset in a bossa nova.
• Dominant seventh (7): a major triad plus a minor seventh — G B D F. Bright on the bottom, restless on top. This is the engine of tonal music.
• Minor seventh (m7): a minor triad plus a minor seventh — D F A C. Smooth and mellow; the chord jazz players reach for on the second degree.
• Half-diminished (m7♭5, ø7): a diminished triad plus a minor seventh — B D F A. Anxious but not frantic; it sits on the seventh degree of every major key.
• Diminished seventh (dim7, °7): minor thirds all the way — B D F A♭. Perfectly symmetrical, so it belongs to no key and can resolve in four directions. Silent-movie villains loved it.

Where each one lives

Harmonise the major scale in four-note chords instead of three and every degree gets its seventh for free — the notes of the key decide which flavour:

Notice the pattern: the two major sevenths sit on I and IV, the minor sevenths on ii, iii and vi, and the one dominant seventh on V. That uniqueness is why a G7 says “C major” so unambiguously: no other chord in the key has that recipe.

Why V7 pulls so hard

Inside G7 (G B D F) hide two notes six semitones apart: B and F, a tritone — the most unstable interval there is. B is the leading tone of C major and wants to rise to C; F is the fourth degree and wants to fall to E. When both move, the tritone collapses into the C and E of the tonic chord. That double resolution is the authentic cadence at full strength, and it is why adding the seventh to V makes the pull to I stronger, not weaker. The half-diminished and diminished sevenths carry the same tritone, which is why they lean toward resolution too.

Inversions: same notes, new bottom

A chord’s identity is its set of notes; which one is lowest is a choice. In root position the root is the bass. Move the root up an octave and the third is on the bottom: first inversion. Move the third up too: second inversion, fifth in the bass. Four-note chords have one more: third inversion, seventh in the bass. Inversions let the bass line walk smoothly instead of leaping from root to root, and they change the colour: G7 with B in the bass (written G7/B, a “slash chord”) slides into C far more gracefully than G7 in root position.

In the lab, build every seventh chord on every root, flip it through its four inversions and read the bass note off the piano. Watch for the dashed tritone arc: it appears only on the chords that pull.