A Dominant chord is the 5^th chord of the scale. For instance, G7 would be the dominant chord in the scale of C. A dominant 7 chord is root, 3^rd, 5^th and a flat 7^th.

A Dominant chord creates tension or instability. Dominant 7 chords beg to be resolved by the tonic chord. Meaning playing a G7 while in the key of C, makes the C chord feel like home. It doesn’t have to be a seventh chord, but the resolution feels much stronger with the seventh.

• E7 resolves to A
• F#7 resolves to B
• G7 resolves to C
• A7 resolves to D
• B7 resolves to E
• C7 resolves to F
• D7 resolves to G

A Secondary Dominant Chord is five steps away from V chord. So if your V chord is G7, D7 would be the secondary dominant.