Oliver Sim and Romy Madley-Croft met at kindergarten when they were three years old, and they’ve been best friends ever since. But it was only when they went on holiday as teenagers that they decided to write their first song together. As Sim explains over the phone from his living room in London, The xx’s unusual question-and-answer vocals – which feature interchanging lead parts from both Sim and Madley-Croft – were developed because as a teenager, he was a little embarrassed to sing in front of his best friend.

“We’ve always sung together,” says Sim, thinking back. “When we were 15 and we started doing covers, it was pretty embarrassing to just start singing in front of your best friend. So we came to an agreement that we would sing together. And then we worked up the confidence to sing by ourselves, even if it was just for a verse.”

After they’d been playing for some time, Sim and Madley-Croft recruited another long time friend, Jamie Smith, to produce their debut album. Smith acts as the counterbalance in The xx, adding darker textures to Sim and Madley-Croft’s minimal pop songs. As Sim explains, Smith is governed by a completely different set of rules, creating spacious beds for Madley-Croft’s laconic guitar lines.

“Jamie has such a different mind to me and Romy. I listen to words and songwriting, whereas Jamie can listen to a song 20 times and not hear a single lyric. He can’t sing a verse to any of our songs,” he laughs. “Jamie understands the logistics of music – chord progressions and arranging songs. I find confidence with him there.”

Once they had established themselves as a three-piece in 2009, The xx recorded their debut album in a garage at the back of their record label, Young Turks, in West London. Being able to record in their own studio gave them space from the outside world – the same space that is evident in tracks like Crystallized, which combine Smith’s electronic beats with sparse vocals and melodic guitar lines.

Each evening, the threesome would arrive at the studio in Notting Hill and work well into the night. “We didn’t need money from our label to record, so we had more time with it,” says Sim. “We weren’t really working to any expectations so if we had failed, it wouldn’t have been such a huge thing for the record label. There was no clock ticking.”

Their first album xx went on to win the Mercury Prize for best British album in 2010 and received three Brit Award nominations. After playing some of the world’s largest music festivals including Coachella, Bonnaroo and Lollapalooza, Sim, Smith and Madley-Croft returned to the studio in 2011 to record their sophomore album, Coexist.

Moving into a new studio in East London, they again separated themselves from the outside world and recaptured the mood from their debut. “The second time around, there were expectations,” recalls Sim. “We have an audience now and if we wanted to play a shitty pub to test out songs, they would end up on YouTube or something.”

It’s a world away from where it all began. But in some ways, not all that much has changed. More than 20 years after meeting in kindergarten, Sim and Madley-Croft still live just a few minutes walk away from each other in South London. There’s tenderness in the way he describes their relationship – a softness that extends into all of their recordings.

Before he hangs up to unpack some boxes in his new apartment, Sim admits that they were never trying to make it big. “You know, it came from the most unambitious place, all of this,” he says. “We were just on holiday and we got really obsessed with music. The best thing is that it’s even better now…with just the three of us.”

The xx are playing at the Hordern Pavilion in Sydney on April 6 and 7. Both Sydney shows are sold out.