G Dominant 7th
7
0 - 4 - 7 - 10Major + minor 7th
About G Dominant 7th
G7 is the chord that will not sit still. Add an F to a G major triad and the chord acquires a tritone — F against B — which the ear treats as a question. In the key of C that question has one obvious answer, and G7 is how nearly every simple song announces it is about to end.
The note that creates the pull
G, B, D, F. The first three are plain G major; the F is a minor seventh above the root and does the work. Between B and F lies a tritone — three whole tones, the most unstable interval in the system — and both notes want to move: B upward to C, F downward to E. Play C major after G7 and you hear them arrive.
Take the F away and the urgency goes with it. That is the practical difference between G and G7: one can wander, the other has somewhere to be.
On the fretboard
The open shape is a G major with the little finger moved from the high E string to the first fret — one finger changes, and the chord changes function. That economy is why G7 appears so early in guitar teaching.
In blues the same chord is played with a barre at the third fret, which frees the hand to add the sixth and ninth above it. The open version rings; the barred version can move.
Where it shows up
As the V7 of C major, which makes it the single most common seventh chord in beginner repertoire. Twelve-bar blues in C uses it in two of its three chords.
In blues the pull is deliberately left unresolved: every chord in the progression is a seventh, so nothing settles, and the form keeps moving. That is why a blues in G uses G7 as home rather than as a departure — the same chord doing the opposite job.