As you’re racing through a thriller or romance novel, you’re not thinking about the fonts or layout on each page. But a designer has spent hours poring over each element on the page to create the most delightful reading experience.

Just ask Leah Carlson-Stanisic, associate director of design at HarperCollins, one of the four biggest publishing houses in the the world. When a manuscript comes across her desk, she considers what font best expresses the content. Historical fiction might warrant a font created in the 1800s. A book about technology might require a more recent sans serif. “It’s 30% experience and 70% intuition,” she says.

But over the past three years, HarperCollins’s designers have put their skills toward a new mission: saving paper. In an effort to reduce the carbon footprint of each book, they’re tweaking fonts, layout, and even the ink used. The goal is to pack more into each page, while ensuring that the pages are as readable as ever. And so far, these subtle, imperceptible tweaks have saved 245.6 million pages, equivalent to 5,618 trees.