Backdoor cadence
In jazz and jazz harmony, the chord progression from ii7 or IV7 to ♭VII7 to I has been nicknamed the backdoor progression or the backdoor ii-V, as described by jazz theorist and author Jerry Coker. This name derives from an assumption that the normal progression to the tonic, the ii-V-I turnaround (ii-V7 to I, see also authentic cadence) is, by inference, the front door. It can be considered a minor plagal cadence in traditional theory (see modal interchange and plagal cadence).
The backdoor progression can be found in popular jazz standards in such places as measures 7 and 8 of the A section of "Cherokee," measures 9 and 11 of "My Romance" or measures 10 and 28 of "There Will Never Be Another You," as well as Beatles songs like "In My Life" and "If I Fell."
The ♭VII7 chord, a pivot chord borrowed from the parallel minor of the current key, is a dominant seventh. Therefore it can resolve to I; it is commonly preceded by IV going to iv, then ♭VII7, then I. In C major the dominant would be G7: GBDF, sharing two common tones with B♭7: B♭DFA♭. A♭ and F serve as upper leading-tones back to G and E, respectively, rather than B♮ and F serving as the lower and upper leading-tones to C and E.
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