Dominant 7th

Notes and formula

The formula is 1 – 3 – 5 – ♭7, or 0 – 4 – 7 – 10 in semitones from the root. A major triad, then a minor third on top: C to E, E to G, G to B♭.

The interval that matters is not the seventh above the root but the tritone inside the chord — the six semitones between E and B♭. Those two notes lean in opposite directions, E upward to F and B♭ downward to A, and both land in the key of F. That is the whole mechanism of a dominant resolution, and it is why C7 sounds like it belongs to F rather than to C.

How to play it

On piano the plain form is C–E–G–B♭. The working form drops the fifth: C in the left hand, E and B♭ in the right. Two notes are enough to state the chord, and adding the fifth back changes nothing.

On guitar the open C7 is x32310, and the movable forms are 8x8998 with the root on the sixth string and x3535x on the fifth. Blues players use only part of the shape — the third and seventh on the middle strings — because that fragment is what carries the sound.

What it does in a key

Build a seventh chord on each degree of a major scale and exactly one comes out as a dominant seventh: the fifth. G7 in the key of C, C7 in the key of F. Hearing a dominant seventh therefore tells a listener which key they are in, because there is only one place it can come from.

Composers exploit this by borrowing. A dominant seventh inserted before any chord makes that chord sound momentarily like a home — the secondary dominant, written V7/V and heard in almost every jazz standard and half the hymn book.

The blues exception

In blues the rule is broken on purpose. All three chords of a twelve-bar blues are dominant sevenths — C7, F7 and G7 in C — which by common-practice logic means the music is in three keys at once. It is not; the flattened seventh is a colour there rather than a signal, and the tension never resolves because it is not meant to.

This is why the same chord reads as unstable in one style and as home in another, and why a jazz musician and a blues musician can disagree about whether a piece has ended.

Substitutions

A tritone is symmetrical: the six semitones between E and B♭ are also the six semitones between B♭ and E. That means C7 and F♯7 contain the same tritone, and either can resolve to F. Swapping one for the other is the tritone substitution, and it is the reason jazz bass lines so often walk down by semitone.

The chord also tolerates alteration better than any other. Raise or lower its fifth and ninth and it stays recognisably a dominant — every altered chord in the catalogue starts here.

Scales that contain it

Mixolydian, the fifth mode of the major scale, fits without alteration and is the default choice. Over a blues the minor pentatonic and blues scale are used against the same chord, which is where the characteristic clash between a minor third in the melody and a major third in the harmony comes from.

When the dominant is altered, players switch to the altered scale or one of the symmetrical scales. All of them keep the third and the seventh — the two notes that make the chord a dominant — and change everything else.

Frequently asked questions