Barbershop quartet music, like ragtime, is a great source of common-tone diminished seventh chords, when it’s time to teach those. It’s also full of parallel tritones in chromatic descent implying root motion around the circle of fifths. It actually has a lot in common with postminimalism: a use of voice-leading so consistently circumscribed that it tends to generate the same consonant (but not always functionally-related) sonorities over and over again. But the style contains one common chord I haven’t seen in any other context: a dominant seventh built on the leading tone, nominally a V7/iii but never resolved as such – rather, a chromatic neighbor-note chord to the tonic triad, given here at the two X’s:
Does anyone have a name for this chord? It often appears between two tonic triads, and the use of the seventh (G, here) helps avoid parallel octaves that might otherwise occur between two B-flats and two A’s. It’s almost identical to a common-tone diminished seventh and serves the same function, only the common tone is done away with, which drops down to the leading tone. The entire assemblage also occurs a perfect fourth higher – i.e., as a D dominant 7th making a neighboring chord to E-flat. Anyone have a name or term for these dominant sevenths used as chromatic neighbors? Anyone ever see them in any other context besides barbershop quartet music?
UPDATE: Allow me to emphasize, since it keeps coming up in the comments, that the entire song is in B-flat major, with no other key ever noticeably implied, and that the chord in question occurs in several varied contexts: quick, drawn-out, passing, neighbor, and so on.