The Standard Bridge

The two major formal units of the pop song are the verse and the bridge (or middle eight). The primary section of a song is its verse, which may or may not include a chorus. The verse is typically heard several times in the course of the song, with lyrical variation normally marking each repetition. The bridge is a secondary section heard sporadically between instances of the verse. It is typically first heard between the second and third instances of the verse, making for an AABA formal pattern. If a fourth instance of the verse is heard, it is usually preceded by a second instance of the bridge, so that an AABABA pattern results. The lyrics of the bridge are usually unvaried from one instance to the next, but there are occasional exceptions to the rule.

The purpose of the bridge is to provide a musical contrast to the verse. In pop songs of the late 1950s and early 1960s, the verse typically begins and ends on I, so an effective means of creating harmonic contrast is to begin and end the bridge with something other than the tonic chord. The IV chord is a natural choice for the beginning of a bridge because its root lies a perfect fifth below that of the I chord on which the verse typically ends and thus makes for a strong I-IV progression between sections. For the end of the bridge, a V chord is ideal, as it likewise produces a catapentic progression (V-I) in the retransition to the verse.

The harmonic contrast is often enhanced through tonicization of the initial IV chord and/or the terminal V chord. Tonicization of initial IV is achieved by adding a chromatic minor seventh to the tonic chord that ends the verse, thereby producing a V⁷/IV chord in the post-cadential turnaround. The terminal V chord is similarly tonicized by a preceding V⁷/V.

This harmonic pattern is perfectly realized in the bridge of the Beatles’ This Boy, heard between the song’s second and third verses:

IV - ♯iioo7 - I

The ♯iio^o7 is a diminished-seventh chord rooted on the raised supertonic (♯2). It occurs with some frequency in the pop music of the late ‘50s and early ‘60s, regularly progressing to I, with its root (♯2) moving up a half-step to the third of the tonic chord (3), its third (♯4) moving up a half-step to the fifth of the tonic chord (5), its fifth (6) descending by whole step to the fifth of the tonic chord (5), and its seventh (1) carried over as the root of the tonic chord. 

 


It’s the carrying over of  the seventh of the diminished-seventh chord as the root of the succeeding chord (represented here in the soprano voice) that gives rise to the term ‘common-tone resolution’ for a progression of this sort. It is to be distinguished from the more classical resolution in which the root of a diminished-seventh chord moves up a half-step to the root of the succeeding chord, as in the soprano voice below:  

 

Note how much more final this viio^o7 - I resolution sounds, even with tonic chord inverted. The ♯iio^o7 - I resolution, by contrast, sounds incomplete and for that reason is rarely used in a final cadence. 

In pop music of the period ♯iio^o7 is more often than not preceded by IV, making for a stock IV - ♯iio^o7 - I progression. 

 This combination of chords implies a 4 - ♯4 - 5 melodic line, characteristically heard in the bass, as above, so that the tonic chord occurs in its unstable second inversion, with a strong propensity to move onward.

An early example of this harmonic structure is found in Bessie Smith’s 1929 recording of Jimmy Cox’s Nobody Knows You When You’re Down And Out:
 

F                  D♯dim^o7/F♯    C/G                   A⁷  

     It’s mighty strange,                  without a doubt,

IV                  ♯iio^o7              I                     V⁷/ii

   D⁹                                                  G⁷                  C

          Nobody knows you when you      down and out
V⁹/V                                                V⁷                   I




Here the IV chord begins a 4-beat hypermeasure that has ♯iio^o7 falling on the unnaccented second hyperbeat (appropriately accompanying the word ‘strange’) and I falling on the accented third hyperbeat. A chromatically altered submediant chord (VI⁷ = V⁷/ii) fills out the fourth hyperbeat, initiating a V⁷/ii - V⁷/V - V⁷ - I, catapentic chain that runs through the next hypermeasure and ends in an authentic cadence. 

The V⁷/ii - V⁷/V - V⁷ - I progression makes a particularly nice complement to the IV - ♯iio^o7 - I progression, balancing its chromatically ascending 4 -♯4 - 5 line (heard in the bass voice below) with a chromatically descending 5 - ♯4 - ♮4 - 3 line (heard in the alto voice):