Traditionally – and especially in the classical era – the dominant 7th chord is a V7 chord in a diatonic chord scale that resolves back to the tonic (or I) chord.
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In modern music however, our ears are used to hearing the dominant 7th chord function in more ways than just a V7 resolving to a I. In blues and pop music, the dominant seventh chord can be used as the I (or home) chord. This is most evident in the blues, where all three chords are often dominant 7th chords.