WHAT holds a pop-soul duo together for 51 years, allows for them to sell 13m albums in the US alone and have six number one singles?

Daryl Hall and John Oates sparkled in the glitter storm of the Seventies, soared in the shoulder-padded, Eighties and even survived the gangster rap attacks of the Nineties.

Now, after a long hiatus the world’s most successful duo are touring again. But what’s the sealant this time around? Cash? Adulation? No, I can’t go for that. There has to be more.

Let’s go back in time. Was there a perfect harmony from the beginning?

The relationship, reveals Oates, began with a bang. Literally. The smaller, darker, half of the duo, who left his Zapata moustache behind for good in the Nineties, tells of their 1968 meeting at a gig in Philadelphia.

“We were in different bands at the time, (The Masters and The Temptones) and ready to promote our singles being played on Philadelphia radio, doing some lip synching,” he recalls.

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“But a gang fight broke out and we leapt into an elevator in a retreat to the ground floor.”

Outside in the alley, having literally dodged a bullet, Daryl Franklin Hohl and John William Oates chatted and found a real commonality.

“We discovered we’d grown up just a few miles from each other,” says Hall, who looks like Robert Redford’s rock star brother.

“We were at the same university, and we learned we were very similar in the music that drives us.

“But while I was into Philly music, John brought his folk music background.”

Hall, whose father was a professional singer and his mother a vocal coach, sang perfectly from an early age .

The very young Oates had been taught the accordion and quickly progressed to guitar. Hall studied Music at Temple University while Oates opted in journalism.

But by the age of 19, both were majoring in becoming pop stars.

The pair formed Hall & Oates. But Hall says their similarities were also complimented by their yin and yang differences.

He emerges, for example, as the more self-assured, confident - and formidable. (His band would later refer to him as Der Fuhrer).

Oates, a college wrestler and tennis player, is considered the more sensitive.

“It was us against the world,” says Hall, reinforcing his determinative streak.

But was it true, as some rock mags have a claimed, that Hall was in fact the leader, and Oates the support?

Oates says the relationship worked from the outset because no one was in charge. Hall would often take lead vocals but when they came up with the classic She’s Gone for example, it was Oates who brought his partner the chorus and they worked on the verse together.

“Sometimes, we do things together, sometimes, not,” he explains.

“We both respect each other’s musical sensibilities, and we have an innate sense of what works.”

Hall agrees. “And part of the success is down to creating songs that are real. Material that’s so often biographical is what we’ve been about.”

So what inspired She’s Gone, the song that every hetero man who has ever been dumped can connect with?

Oates grins as he rewinds; “I met this girl in a coffee bar in the middle of the night in Greenwich Village.

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“She was wearing a pink tutu and cowboy boots and was something of a wild child. We really hit it off. But on New Year’s Eve when I invited her round to my apartment she didn’t show up.”

The tragic experience was enough for Oates to take his near suffocating sorrow, throw some chords around it, and take the haunting chorus round to his songwriter partner to work up a verse. Thank god for misery?

“Yes,” says Oates, laughing. “It always helps. Incidentally, did he ever hear from the pink tutu girl again? “She popped up over the years, but nothing happened.”

Still, Hall & Oates had each other. And in 1972 released their first album, Whole Oates. It was “too folky” said the critics, but the follow-up Abandoned Luncheonette, a blend of Philly R&S and Motown was critically acclaimed.

The next album however, War Babies, says Rolling Stone Magazine, “alienated everyone.”

What this revealed was that Hall & Oates were never going to write to prescription. In 1975, the Silver Album (with their much talked about cover photograph which made them look like apprentice drag queens)) produced the hit Sara Smile, written for Hall’s long-term girlfriend Sara Allen.

But the duo then endured a period when their albums were received to the sound of silence.

In 1980 however the Voices album had hits fly out of it like a wonderfully broken juke box. The likes of Kiss On My List, You Make My Dreams, and I Can’t Go For That stormed the US and UK charts.

And they lived a life of world class, pop star indulgence. The duo lost most of their money to sillyness and the sharks in the record industry.

But they sued the record company, won back royalties and ownership of their songs.

After a hiatus, they got back together. And both say it’s incredibly easy to pick up where they left off, whether it’s at Caesar’s Palace or the Hydro.

. The pair say they have endured because they are more like brothers than friends.

Yet, perhaps the main reason Hall & Oates are still appearing together aged 72 and 71, is they are unabashedly appreciative.

“Lots of people would dream of having this opportunity and I’m all too aware of how precious it is,” says Oates. “For us to be able travel the world and have this 50 year career, well that’s great.”

Daryl Hall & John Oates, the SSE Hydro, with KT Tunstall, May 1.