Dominant seventh chord
The dominant seventh (V7) is the dominant triad with a fourth note added a third higher: in C major G–B–D–F, written G7. It sounds tense and wants to return to the tonic.
Take the dominant G: G, B, D. Add one more note, skipping a key again: F. Why “seventh”? From the bottom G up to F is seven letter names, a seventh. Hence the 7 in the chord letter G7. The chord has four notes, three thirds stacked up.
Two notes create the tension of V7. B, the seventh degree of the scale, strains a half step up to C. F, the fourth degree, falls a half step down to E. When both arrive, the tonic C–E–G sounds. That is why a cadence in which V7 returns to I is the most final: F and B lead straight home.
In songbooks G7 is one of the most common chords: the 7 after a letter adds the fourth note. The left hand often plays V7 in an inversion so the fingers stay near C: B–D–F–G. In other keys V7 is built on degree V: D7 (D–F♯–A–C) in G major, C7 (C–E–G–B♭) in F major.
What it sounds like
V7 sounds tense and unfinished – like a question that the tonic answers. It is what makes an ending firm.
Where it is taught: the studio’s interactive textbook, chapter 11, “Triad inversions and the dominant seventh” and chapter 19, “Lead sheets: melody and chord letters”.