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:
