There are 5 dominant 7th shapes in the open position. They are C7, A7, G7, E7, and D7. All other dominant 7th chords must be played with a barred version of one of these chords.
C7
G7
D7
A7
E7
Dominant 7th chords are called “dominant” because they are a common substitution for the dominant chord in a song. The dominant chord being the 5th of the tonic or V.
A dominant 7th chord creates a little tension and they sound like they want to resolve to the tonic chord creating a V-I progression.
Listen to the examples below. Hear how the the dominant 7th chord is resolved by the tonic chord.