In an email, Mr. Nowels wrote: “Romy is a wonderful, natural singer and songwriter. She’s a strong intuitive musician which informs her songwriting. Her voice has a quiet intelligence. She writes lyrics from a personal and soulful place. She’s one of the few guitarists these days who has an instantly recognizable style — that’s almost impossible to achieve. She played through my little amp with some reverb, and there was that sound!”

The camps, Ms. Madley Croft said, “taught me to trust my instincts. Everybody moves so quickly, works so quickly, that it’s kind of like oiling up your joints. But I was not quite pouring out my soul in that music. So I was really happy to come back with the boys and just get really cathartic again.”

But they were working differently this time. Mr. Sim and Ms. Madley Croft sometimes worked together, trading melodies and bits of lyrics, as in the songwriting camps, and no longer worrying who wrote what. Mr. Smith channeled some of his D.J. expertise into the new songs — although, he said, “It still sounds like there’s space compared to what you hear on the radio these days.” And instead of staying cloistered, the band took some of its new songs on the road in 2014, playing small gigs around the South.

The band’s early reticence was no pose. Mr. Sim and Ms. Madley Cross, both 27, met at the musically oriented Elliott School in the London suburb of Putney and wrote songs, at first, just for themselves. “We didn’t think anybody was going to hear them,” she said. “They were like a diary for us. But then people connected with that.”

They were so uncertain about their lyrics and singing, she added, that even while they were making their first album they exchanged lyrics by email, not in person. By then the xx also included two fellow students: Mr. Smith, 28, and the guitarist Baria Qureshi, who left after the xx’s first album.