Seventh chords

Add F on top of G-B-D and you get G7, a chord whose B wants to rise to C while its F wants to fall to E.

One more third

Stack one more third on top of a triad and you get a four-note seventh chord, named for the interval from the root to the new top note, a seventh. On the staff it looks like a taller snowman: four notes on four neighbouring lines or spaces. The quality of a seventh chord depends on two things, the triad underneath and the size of the seventh. Five combinations are common, and each has a name and a symbol.

Notice the symbols carefully, because they are easy to mix up. C7 on its own does not mean a major seventh: it means the dominant seventh, a major triad with a minor seventh (C E G B♭). The major seventh needs "maj7" (C E G B). The two diminished types differ only in the top note: the half-diminished has a minor seventh and is written with a slashed circle, ø; the fully diminished has a diminished seventh, three half steps above the fifth, and is written °7.

Spelling a seventh chord uses the same steps as a triad. Write four letters a third apart (G, B, D, F), think of the major key signature of the root, and apply its accidentals: G major has F♯, so the G major seventh chord is G, B, D, F♯. Lower the seventh and you have the dominant seventh G, B, D, F. Lower the third as well and you have the minor seventh G, B♭, D, F. The letters stay fixed while the accidentals change.

Seventh chords in a key

Build a seventh chord on each degree of a major scale and the qualities fall into a fixed pattern, the same in every major key. In C major: Cmaj7 (I⁷) and Fmaj7 (IV⁷) are major sevenths; G7 (V⁷) is the only dominant seventh; Dm7 (ii⁷), Em7 (iii⁷) and Am7 (vi⁷) are minor sevenths; Bø7 (viiø⁷) is half-diminished. That makes the dominant seventh a fingerprint: in a major key it appears naturally on only one degree, 5, so hearing it points straight to the key.

Why the dominant seventh wants to move

G7 contains two notes that lean toward C major. B is the leading tone, a half step below C. F is the seventh of the chord, a half step above E. When G7 moves to C, B usually rises to C and F usually falls to E, each by a half step. B and F are also six half steps apart, a tritone, an interval that sounds strongly unstable, and the resolution turns it into the stable third C-E. Try it on the keyboard below: play G B D F, then C E G, and listen to the two half steps.

Seventh chords can be inverted like triads, with one extra position because there is one extra note. With the seventh in the bass the chord is in third inversion. Figured bass shortens the four positions to 7, 6/5, 4/3 and 4/2, so V⁶₅ means a dominant seventh with its third in the bass. Chord symbols show the same thing with a slash: G7/B is a dominant seventh on G with B in the bass.