A librarian has stumbled upon a rare 540-year-old page, ripped from a Medieval priests’ handbook, in a library at the University of Reading, United Kingdom.

According to experts this page comes from a medieval priests’ handbook that had been printed by William Caxton, who introduced the printing press to England. The paper, dating back to late 1476 or early 1477, was found hidden in an archive. The double-sided page has black-letter typeface and red paragraph marks that gave it away as an early western European printing. The page had previously been pasted into another book for the undignified purpose of reinforcing its spine. Experts figured out that in 1820 a librarian at the University of Cambridge saved the page from the book spine but apparently didn’t realize its worth. The 15th-century leaf then ended up in a private collection that was purchased by the University of Reading 20 years ago.

(after University of Reading & Live Science)