Fore-edge painting; The Royal Kalendar, and Court and City Register, for England, Scotland, Ireland, and the Colonies, for the year 1849.

A book designer is responsible for every aspect of the way a book looks and feels, from the jacket art and title page down to the page numbers and line spacing. Even the edges of the book have to be considered—you’ve probably encountered a book with deckle edges (where the pages are left untrimmed, creating a rough edge) and anyone who’s ever read a Bible is likely familiar with gilded edges.

It’s far less common, however, to encounter a design on the edge. The Boston Public Library’s online exhibition of books with fore-edge paintings contains more than 200 examples of this seldom-seen art. According to the exhibit’s introductory essay, the fore-edge painting was first developed in the sixteenth century by a Venetian artist named Cesare Vecellio, and it became most popular in the late eighteenth and early nineteenth centuries in England. Many of the paintings feature pastoral scenes related to the books’ settings, but others contain portraits of their authors or characters, and some even depict moments of action.

Most intriguing was the technique discovered by an English bookbinder named Samuel Mearne in the seventeenth century which allowed the fore-edge paintings to be “hidden” in the book.

“Imagine a flight of stairs, each step representing a leaf of the book. On the tread would be the painting and on the flat surface would be gold. A book painted and gilt in this way must be furled back before the picture can be seen.” (Kenneth Hobson, 1949). This is how a fore-edge painting works. When the book is closed, you do not see the image because the gilding hides the painting. But, when you fan the pages to show the painting at its best and hold them between your fingers or in a display press, the colorful picture appears as if by magic.

Today you’re unlikely to encounter such a painting at your local bookstore, but the technique hasn’t disappeared entirely: book design legend Chip Kidd’s 2001 novel The Cheese Monkeys is printed with a hidden message on the fore-edge (which spells “Do you see?” when the pages are fanned one way and “Good is dead” when they’re fanned the other), and a new YA novel from the James Frey factory, I Am Number Four, has a message of its own (unfortunately, not “You got scammed”):