The dominant 7 chord is the most common of all the 7 chords. As the title suggests, it is built on the 5th degree (called the dominant) of the major scale and normally resolves to 1 or 6 minor.
Unlike some of the 7 chords that simply add color to the sound of major or minor chords, the dominant 7 chord creates a strong harmonic pull which sends the chord progression in a definitive direction. This has much to do with the dissonance it creates.
Look closely at any dominant 7 chord and you will find a tritone interval between the 3rd and the 7th of the chord. Called the most unstable interval, the tritone can only be dissolved by resolving the dominant 7 chord to the 1 major.
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