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"I Heard It Through the Grapevine": How the Chord
Progression Works
Below are the two interpretations. The chord map for "I Heard It Through the Grapvine" reveals no fancy extended chords, just two ordinary seventh chords and two ordinary minor chords.

FIGURE 99: Chord Map of “I Heard It Through The Grapevine” (Words and Music by Norman Whitfield and Barrett Strong, 1967)
From a melodic standpoint, the minor third interval relationship with the tonic means the song is clearly in the minor mode. So the second of the above two interpretations is technically more correct, even though the first interpretation is harmonically simpler in that it does not have a chromatic chord.
Either way you care to interpret this chord progression, it’s ingenious and ear-grabbing.