C Dominant 7th
7
0 - 4 - 7 - 10Major + minor 7th
About C Dominant 7th
C7 is C major with a flat seventh, and on guitar it is the first seventh chord that asks for a stretch. The B♭ sits on the G string at the third fret while the hand is still holding the C shape below it — a small reach that catches players who learned C with three fingers.
The notes
C, E, G, B♭. The B♭ is the note the key of C does not contain, which is the point: a dominant seventh built on the tonic pulls the music out of its own key and toward the subdominant. C7 wants to go to F.
This is why C7 rarely appears in a plain song in C major. It shows up when the music is about to move — the moment before a chorus changes key, or the first bar of a blues.
On the fretboard
The open shape uses four fingers if the low G is included, three otherwise. The B♭ replaces nothing — it is an addition, and the hand has to make room.
The barre version at the third fret, A-string root, is the more common form in blues and jazz, because it moves and because the open shape is awkward to strum quickly.
Where it shows up
As the V7 of F major, and as the I7 of a blues in C. In the second role it is the chord the whole form starts from, which is unusual: most keys build the blues on a chord that is easy in open position, and C is not.
It also appears in songs that modulate from C to F — the flat seventh is the hinge, and the ear follows it without noticing the key has changed.