A dominant 7th chord (usually just called a 7th chord) is created by adding a flattened 7th degree to any major chord. For example, a C7 chord contains the notes C, E, G and Bb (1, 3, 5 and b7). Here are some common open dominant 7th chord shapes. These are particularly common in Blues, Bluegrass and Jazz.
D7
E7
F7
F#7
G7
Ab7
A7
B7
C7
Db7
Eb7
Bb7