Tegan and Sara Quin received their first piece of “fan mail” when they were 17. It was the summer after they had graduated from high school, and the twins had decided to pursue music instead of going to university. So when an aunt they were visiting asked them to perform for her and some friends, they pulled out their acoustic guitars and sang, as they’d done at open mics and house parties, a few growling songs about their identities and uncertain futures, their dueling perspectives united by just similar-enough voices.

The next day, they woke up to a three-page, front-to-back single-spaced letter from a friend of a cousin, a guy in his 20s, who congratulated them on the performance... and then proceeded to list all the ways they could be better: chords they should use, techniques they should consider, changes they should make to their singing styles. He thought he was being helpful: “I think you guys have a real chance or else I wouldn’t be writing this.”

Sara uncovered the letter last year amid old journals and lyrics while researching her and Tegan’s upcoming memoir, High School, out now. Told in alternating chapters from each of their perspectives, the book revisits the traumas and transformations of their acid-fueled dirtbag teen years, during which they discovered music and navigated their sexuality as young queer women in mid-1990s Calgary. Now 39, Sara hadn’t seen the letter in decades. But the familiarity struck her: How many times over the years had men—friends, musicians she admired, random people she had sat next to on airplanes—given her unsolicited advice about how they could make their music less simple (that was the word they always used) and more interesting to them? But another question nagged at her: Why—after eight studio albums, a million records sold, a handful of awards—had she believed them on some level?

Tegan, the older of the two by eight minutes, didn’t care about the letter—she never put much stock in that kind of criticism anyway. But Sara clung to it. When she revisited some of their early press clippings, she noticed that she’d volunteer comments about how rudimental a musician she was, tearing herself down before anybody else could. “I was like, oh shit, we were good—fuck these motherfuckers, why did I adopt this narrative?” Sara says one summer afternoon over drinks in SoHo; she’s having a Tom Collins, Tegan a sparkling water with lemon and lime. “I believed that we were not good because a few people said we weren't good, and that was how I felt for 21 years.”

(Tegan) T-shirt, $110, by Jenni Kanye / Jacket, price upon request, by Jacquemus

The letter got one thing right, though: They did have a real chance. The following year, Neil Young’s longtime manager signed them to his label. Over the next two decades, they smoothed out the rough edges of their early work into slick new-wave hooks dressed up in an indie-rock package, then dispelled all of that on 2013's Heartthrob, a glossy Top 40 makeover that wound up on numerous critics’ year-end lists. Even more special was the culture they’ve built around the band: The stories they’d tell between songs in their early years inspired a Deadhead-like devotion among fans who’d travel around the world to record them. And long before they shared their journey to becoming artists in High School, they offered listeners a different kind of how-to manual through multiple documentaries, fake talk shows, web series, and batches of homemade demos, turning Tegan and Sara into not just a band but a content house ahead of its time.