Endpapers are another part of the book that is both functional and decorative. We’re all familiar with the pretty marbled papers that can be found on many older books on the inside of the front and back cover—one half is glued to the inside of the cover and the other half forms the first leaf of the book (the flyleaf), which is turned over to reveal the title page. A splash of psychedelia that indicates you are leaving the world proper and entering some other kind of space.
Endpapers may be stitched onto the text block or tipped on (adhered to the front page of the text block by a narrow strip of glue down the spine edge), or both.
Pasting one half of the endpaper to the cover helps to counteract the curvature caused by the covering material on the front, adhesives generally causing a contraction on the side to which they are applied. The endpapers also help to disguise the turn-ins—the edges of the covering material (eg leather) that have been wrapped around the boards. They disguise the mull and any other spine linings that have been applied, and help to attach the cover to the text block. In some cases, the endpapers are just about all that are holding the two together.
All sorts of papers have been used to make endpapers, of course—marbled papers, plain papers, gilt papers, paste papers, moiré cloths and many more. There is no end to their variety.