If you went to middle or high school in the U.S. in the last few decades, chances are you’ve read To Kill a Mockingbird, Harper Lee's now-classic novel (which was adapted into a now-classic film) about racial injustice in the South. Even if you grew up far-removed from Jim Crow laws, you probably still understand its significance; in 2006, British librarians voted it the one book every adult should read before they die. And yet the novel, while considered an instant success, wasn’t always destined for its immense fame, as we learned from the Vox video series Overrated. In fact, its status in the American literary canon has a lot to do with the format in which it was printed.

To Kill a Mockingbird came out in paperback at a time when literary houses were just starting to invest in the format. After its publication in 1960, To Kill a Mockingbird was reviewed favorably in The New York Times, but it wasn’t the bestselling novel that year. It was the evolution of paperbacks that helped put it into more hands.

Prior to the 1960s, paperbacks were often kind of trashy, and when literary novels were published in the format, they still featured what Vox calls “sexy covers,” like a softcover edition of The Great Gatsby that featured a shirtless Jay Gatsby on the cover. According to a 1961 article in The New York Times, back in the 1950s, paperbacks were described as “a showcase for the ‘three S’s—sex, sadism, and the smoking gun.’” But then, paperbacks came to schools.

The mass-market paperback for To Kill a Mockingbird came out in 1962. It was cheap, but had stellar credentials, which appealed to teachers. It was a popular, well-reviewed book that earned Lee the Pulitzer Prize. Suddenly, it was in virtually every school and, even half a century later, it still is.

Learn the whole story in the video below from Vox.