Few contemporary pop singers render desire so expressively as Carly Rae Jepsen and Charli XCX. There's an elasticity to their syllables, a whoosh and snap to how they form words behind the microphone. For Charli, desire manifests in a guttural zone as she striates vowels from her diaphragm, like in the punctuative "no no no no" on "Boys." For Carly, wanting sounds breathier, a sliver of a sigh haloing the end of a word. That the two would eventually collaborate seemed inevitable. After all, they've both worked with PC Music producers — Carly with Danny L. Harle and Charli with A.G. Cook, as well as PC Music associate Sophie — and they both share an ecstatic ethos about the potentialities of pop music in its purest form.

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Their new song together, "Backseat," marks a turn for both of them. The first track off of Charli's new Pop 2 mixtape, "Backseat" isn't the rapturous automotive makeout anthem its title might suggest at a glance. Sure, there's kissing (in Carly's verse, naturally), but it's the hesitant, ambivalent kind. Rather than singing from the blush of a new flame, Carly and Charli are stuck in stagnant relationships. Eighteen months in, and they're starting to wonder if going solo might be a better use of their time than sticking around with their partners. "Year and a half, are we in love? / I'll never know / All that went wrong, what a mistake," Charli sings through sheets of Auto-Tune. It's like she's trying to sing her way out of a monogamous chokehold into the freedom of being alone.