Watch the Alban Berg Quartet perform the first movement of Beethoven's String Quartet, Op 18, No 1

What makes a string quartet more than the sum of its parts, more than four breathtakingly gifted musicians playing four priceless instruments? The answer is something far less tangible than music. No, without the peculiar bonds and excruciating tensions that lie at the heart of a chamber foursome, they might as well play Hot Chip as Haydn.

After nearly 40 years the Alban Berg Quartet are bowing out at the top of their game, with a farewell London season beginning next week. In 2005 Thomas Kakuska, the ABQ's original violist, died of cancer. “We were 25 years together,” says the cellist, Valentin Erben, who made the decision to quit, supported by his fellow Albans.

“It was a strange thing when Thomas died,” he goes on. “All of us wanted to go on and he wanted us to go on.” Kakuska nominated a pupil, Isabel Charisius, as his successor in the quartet. “And Isabel does an incredible job,” Erben continues. “But there was a big rupture in our hearts. She could do, the best one could do but there has been a break.”

The break Erben describes occurred in the bonds that fuse together every great quartet: the Endellions, celebrating their 30th anniversary next year, the Kronos Quartet, 35 years and counting, or even the Beaux Arts trio, another ensemble retiring this summer after almost 50 years. Born of a delicate balance of personal chemistry and musical empathy, the dynamic within a quartet is elusive to an outsider. Even insiders struggle.

“It goes maybe even further than friendship,” suggests Erben, who prefers the German word Kamaradschaft to describe the ABQs' relationships. “We can have a fight and hate each other sometimes but this cannot change the fact that we are deeply related to each other.”

Erben is not exaggerating when he says “related”. Krzysztof Chorzelski, violist with the Belcea Quartet, is an only child and describes his three colleagues as siblings. Others liken the quartet relationship to a marriage, albeit of four people.

However they're defined, these bonds are vital to a quartet. For one thing, they are constantly tested. Rehearsals are a battleground as musical ideas are fought over with gladiatorial passion. “Our moments of greatest tension are in rehearsals,” says David Waterman, cellist with the Endellions. By its very nature a quartet is a powder keg: four independent virtuosos bursting with fervent musical opinions. “It's less clear that you should compromise on the music. You find yourself willing to go to the grave if it's played allegro not allegretto.”

Erben tells of an almighty ABQ barney on tour in Toronto, resulting in one member storming out - not just of the rehearsal but of the group. “So the three decided to go for lunch. We chose one restaurant we like. We went in and he was sitting there. So of course we have to laugh: an argument and then we find out we agree.”

No player would relinquish this conflict; artistic tension is the lifeblood of a quartet. The American Guarneri Quartet, retiring next year after 45 years, “thrives on disagreement and criticism”, says its first violinist, Arnold Steinhardt. If the best ensembles didn't constantly meld their individual ideas and disparate opinions into a unified interpretation - something approximating musical alchemy - we wouldn't experience moments of glorious Beethoven and Bartók. The interesting part is how a quartet resolves the balance between individuals and the group.

With no conductor to pull rank, democracy isn't the solution. Majority rule can never work because the player with the lead melody must feel comfortable. Beyond this, different quartets have different tricks. The Albans still abide by Kakuska's “Shut up!” rule: if two players are arguing with no end in sight, a third can call time out. Inside the Endellions, warring players must try another's musical idea, “to their satisfaction, not with bad grace,” Waterman adds.

Open-minded, respectful, flexible, patient: the essentials of a quartet are the hallmarks of a marriage handbook. But there is no Relate for troubled quartets, and when they go bad they can go very bad. Notoriously, when three members of the American Audubon Quartet dismissed their first violinist by letter, he sued for damages, to the tune of $611,000.

Passions run high in a quartet, sometimes spilling off the music page. Krzysztof Chorzelski and Corina Belcea-Fisher, of the Belceas, dated then split, but, incredibly, managed to avoid a bitter Abba-esque meltdown of the group.

Quartets are nothing if not intense, travelling, rehearsing, performing and living together. And this claustrophobia is all part of the driving tension. They just have to know when to create space.

The Colorado-based Takács Quartet are now in their third decade, with a changed line-up. On tour, its members never sit next to each other on planes. They also take care to book hotel rooms far apart, “for the straightforward reason that we don't want to hear each other practising,” says the first violinist, Edward Dusinberre. “You've had a flight, you want a nap and then the guy next to you starts practising. Not only do you hear him so you can't sleep but you feel guilty that you're not practising yourself.”

Dusinberre teaches various young quartets and he senses a shift towards part-time ensembles. Members of the Quince Quartet, for instance, juggle various solo and ensemble projects. The more established Zehetmair Quartet tours just one programme a year. “I think we were lucky to do things full time because that creates a certain intensity to the work,” says Dusinberre, questioning whether these more flexible approaches may dilute the quartet dynamic.

So is a quartet still for life? Or is it becoming a high-profile stepping stone to a solo career? Not according to new kids on the London scene, the Brodowski Quartet. “This is such a special way of music-making,” says the 26-year-old violinist Catrin Win Morgan. “Playing with the same people all the time, you get to a certain level of understanding. And there's so much wonderful music for us to learn that 15 years doesn't really seem that long.”

There may be some four decades between the Alban Bergs and the Brodowskis but on this point they agree. “I've done this for 37 years and it was such a rich time in every respect; the human and the artistic,” Erben reflects. “I'm grateful that I have lived it. Now there is space for something else.”

The Alban Berg Quartet's London farewell season begins at the Queen Elizabeth Hall, London SE1 (0871 6632500), on February 14

Foursomes to watch: up-and-coming string quartets

Elias Quartet: Sheffield-based but hail from France, Scotland and Sweden. Formed in 1998 and won every quartet award going at the Royal Northern College of Music. Praised for their fire and intensity.

Quince Quartet: All-Brit quartet; one girl, three boys; formed 2003. Studied with the ABQ, Takács and Endellions. Repertoire from Purcell to Tippett.

Ebène Quartet: French, wear black, très sophistiqués. These BBC Radio 3 New Generation artists are praised for their musical understanding.

Pavel Haas Quartet: Prague-based; two boys, two girls. Won the 2007 Gramophone Chamber Music Award for their debut CD of Haas and Janácek.

Cremona Quartet: Italian, formed in 2000 and already praised for a “mature, lyrical” sound. Perform core rep as well as contemporary Italian composers.

Brodowski Quartet: London-based, founded in 2005 from two halves of two competing quartets - the winner and runner-up. The Times described their recent concert at the Purcell Room as “open and honest”.

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