A fore-edge painting is decoration painted on the edges of a book’s pages for beautification. The fore-edge of a book is the fourth edge - not the spine, the top, or the bottom edge, but the outside edge that a reader would use to thumb through the pages. Originally, this edge was only used for identification, to more easily spot a book when the edges faced outward.
Around the 16th century, an Italian artist named Cesare Vecellio (cousin of celebrated Renaissance painter Titian) began to use the fore-edge of books as a canvas, and took the opportunity to make books more beautiful. The first instances of decorative fore-edge paintings were applied outright on the edges of the leaves, easily visible to anyone who cared to look when the book was closed.