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C and G Chords: G7 Chord Progression
The moment your brain hears a single V7 chord, without any other musical reference, without any reference whatsoever to the tonic chord or even the tonic note—the instant that V7 chord sounds, your brain knows where the dynamic centre is. It knows what the key is.
When the seventh is added to the V chord, the chord’s name changes from the dominant chord to the dominant seventh chord.
Try that little experiment with the C and G chords again, but this time, substitute
G7 for G, like this:
C – G7 – C – pause – G7 – C – G7 – pause – G7 – C – G7
Adding that seventh makes all the difference in the world. There’s no ambiguity whatsoever. The key can only be C major.