The paper, often different in color from the book block, that covers the inside of the front and back board and makes the first and last page of a hardcover book.
Strips of paper or cloth webbing or tapes attached to the book block are glued to the boards of a book. This is rather unsightly, especially when the bookbinding is done by hand, instead of the mass produced machine bound books. Also, something is needed to hold down the tapes down. In addition, something is needed to join the book block to the boards, to provide a stronger hinge. The endpapers do all this.
In addition to the structural value of the endpapers, they also provide a visual transition from the cover to the contents of the book. Until the end of the 19th century, endpapers were usually done using marbled paper. Since then, they have usually been paper of a single color, or with a printed pattern.