F Dominant 7th
7
0 - 4 - 7 - 10Major + minor 7th
About F Dominant 7th
F7 is the chord that makes a blues in B♭ possible on guitar, and the one that teaches a beginner what a barre really costs. It is F major with a flat seventh, held at the first fret, where every string is under maximum tension and the frets are furthest apart.
The notes
F, A, C, E♭. The E♭ is the seventh, and it sits a semitone below the open E string — close enough to be confusing when learning, far enough to change the chord entirely.
The tritone between A and E♭ is what gives F7 its pull, and in the key of B♭ it points home. Outside that key it is the sound of a blues that has just left its tonic.
On the fretboard
The full barre at the first fret is standard. A four-string version starting at the D string drops the barre and most of the difficulty, at the cost of the low F.
At the eighth fret with the root on the A string the same chord is comfortable, and that is where most players actually put it once they have the choice.
The chord is also the reason many players finally learn the first-fret barre properly: F major can be faked on four strings, but F7 in a blues needs the low root to carry the shuffle, and the shortcut stops working.
Where it shows up
As the V7 of B♭ major, and as the IV7 of a blues in C. In the second role it is unavoidable: a twelve-bar in C uses C7, F7 and G7, and F7 is the middle one.
It also appears in the key of F itself as a tonic seventh, when the music is about to move to B♭ — the same hinge that C7 provides for a song in C.