Sometimes it's the most profound part of a song, or the moment that interrupts a narrative's confident surface. Often, it's a backing up, a taking stock, a few seconds of reflection, poignant or even painful.

The bridge -- also known as the "middle-eight" for its eight bars -- connects a chorus to a verse; it's associated with Tin Pan Alley and Lennon-McCartney. Arlen-Harburg's "Over the Rainbow" ("Someday I'll wish upon a star ...") and the Beatles' "Yes It Is" ("I could be happy / with you by my side ...") have great ones, the first rueful, the other assertive.

But its roots are far older, and the bridge has persisted as a key songwriting structure despite the attack on convention by punk, grunge and indie.

While some bands today reject the bridge and its formal, polished connotations, it continues to be a key to emotionally resonant and musically organized songwriting: Commercial songwriters use them as elements in a formula, but a handful of today's best musicians rely on the bridge nearly as artfully as Cole Porter.

"I sort of regard the bridge in a magical way," says Aimee Mann, the critically respected singer-songwriter. "It separates the men from the boys. It's a mark of hanging in there, finishing the job, making sure it's a real song."

Mann admires the use of bridges by songwriters such as Jimmy Webb, Randy Newman, Jon Brion and Elliott Smith. "I like the bridge because it's like the counterpoint, 'On the other hand.' I think the best version of this was Lennon and McCartney. They wrote bridges for each other's songs, so you would have a totally different approach," both emotionally and lyrically.

The song "We Can Work It Out," for instance, has a typical McCartney optimism to it, until the bridge, thought to be by Lennon, becomes darker, more dubious, and more hurried on the bridge -- "Life is very short, and there's no time ..." Says Mann: "It's like the devil's advocate."

"Sometimes the song feels like it needs to go somewhere else," says Richard Thompson, who wrote a harrowing bridge for "Walking on a Wire" and a nostalgic one for "Al Bowlly's in Heaven."

"The art of the bridge is that it's an exciting place to go, and the unexpected can result."

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In favor and out

The bridge goes back to one of the earliest blueprints in Western music: sonata form, which was codified in the late 1700s and serves as the foundation for not just the piano sonata but the symphony and concerto. In some cases, the bridge "modulates," changing keys to give the piece a new, sometimes darker, emotional flavor -- as does the related sonnet, with its volta, or "turn," where an ode to a beloved can turn melancholy.

But in much of the music of the 19th and early 20th centuries -- the rural music of Europe or North America, from 12-bar blues to folk and country -- the bridge never took.

In the 1930s and '40s, classic American songwriters such as Porter and the Gershwins used the bridge to bring refinement to their songs -- songs adapted as jazz numbers by Charlie Parker, Miles Davis and others.

In the early '60s, the bridge was reborn with the British Invasion. As song form stretched out in the '70s, structure fell out of favor, but even punk bands the Clash and the Buzzcocks -- and post-punks such as XTC and the Smiths, for which the bridge was the place for rhetorical questions -- wrote concise and effective middle-eights. The Sex Pistols' "God Save the Queen" has a bridge.

Music critic Ira Robbins points out that bridges often find a new vocalist taking over for a few bars, whether it's Keith Richards in Rolling Stones bridges, Pete Townshend for the Who, or Mick Jones for the Clash. Sometimes they provide "a 90-degree turn in the middle of the song."

By the '90s, alternative and indie rock were busy deconstructing the rock song or adding layers of distortion. Nirvana and "lo-fi" groups such as Sebadoh and Pavement couldn't be bothered with structure; post-rock groups such as Tortoise weren't interested.

But Benjamin Nugent, whose book "Elliott Smith and the Big Nothing," which comes out in October, says the bridge is on its way back.

"What a bridge does, generally, is interrupt the beat that a dance song depends on, so for a long time it wasn't the most desirable thing to have," he says. "But starting in about 2001, people had been so saturated with grooves -- whether in dance music or teen-pop -- that they started yearning for clearly structured songs and vocal performances that don't embellish the songs much. So what's happened is a return to the Cole Porter-Burt Bacharach-Beatles elements of a song."