Why are Nashville’s stars touring with the survivors of One Direction?

Ask me anything about the boots Kacey Musgraves wore in concert on Sunday night — I spent the evening watching the Texas singer’s every footfall, waiting for her to step over the invisible line that separates country music and pop. Would a little buzzer go off, like at the bowling alley when your shoes slide too far?

Of course, there is no such line. In music and on maps, borders are just imaginary demarcations used to control people by keeping their money, ideas and DNA separate.

That said, it still feels significant that Musgraves, a “country singer,” is opening for Harry Styles, a “pop star” on an extensive tour of “the United States” this summer. And it looks like a trend, too. Like Musgraves, Maren Morris is preparing to launch a big tour alongside a One Direction graduate, Niall Horan, while Cam, the California balladeer behind 2015’s “Burning House,” will spend her summer nights opening for British soul-guy Sam Smith.

Get in where you fit in, right? Ever since the excommunication of the Dixie Chicks in 2003, country radio hasn’t been all that hospitable to women - let alone women interested in broadening the shape of the genre with new sounds, fresh styles and gently progressive lyrics. So if you think Musgraves, Morris and Cam are out evangelizing for Nashville this summer, think again. Musgraves certainly didn’t make Sunday’s opening set at Washington, D.C.’s Capital One Arena feel like an invitation back to the ponderosa. Instead, she told the crowd she felt as if she had discovered “a second home.”

Morris and Cam have drafted more formal applications for dual citizenship, in the recording studio and in songwriting sessions. Morris has been enjoying a 20-week ride on the Billboard Hot 100 with “The Middle,” a high-def collaboration with the EDM producer Zedd. (Strangely poetic, it’s a song about seeking pleasure through compromise.) And before that, she and Horan recorded a softrock duet together called “Seeing Blind.” As for Cam, she has co-written some of her finest songs with the super-producer Jeff Bhasker, and last year, she helped pen “Palace,” an understated ballad for Smith’s most recent album.

Musgraves’s connection to her touring partner, however, seems more ideological than musical. Both she and Styles are big boosters for LGBTQ rights, and “Follow Your Arrow,” the most heartstunning cut from Musgraves’s 2013 debut album urged listeners to kiss whomever they felt like kissing.