Wikipedia
Fore-edge painting
In order to view the painting, the leaves of the book must be fanned, exposing the edges of the pages and thereby the painting. Another basic difference is that a painting on the closed edge is painted directly on the surface of the book edge (the fore-edge being the opposite of the spine side). For the fanned painting the watercolor is applied to the top or bottom margin (recto or verso) of the page/leaf and not to the actual "fore"-edge itself.