What is the double sheet of paper partly glued to the inside of the cover of a book called?

The endpapers or end-papers of a book (also known as endsheets) are the pages that consist of a double-size sheet folded, with one half pasted against an inside cover (the pastedown), and the other serving as the first free page (the free endpaper or flyleaf). Thus, the front endpapers precede the title page and the text, whereas the back endpapers follow the text. Booksellers sometimes refer to the front endpaper as FEP.

Before mass printing in the 20th century, it was common for the endpapers of books to have paper marbling. Sometimes the endpapers are used for maps or other relevant information. They are the traditional place to put bookplates or an owner's inscription.

As of 2010, there are many styles of endsheets or endpapers that are specifically designed for use with different bindings. For example, endsheets reinforced with cloth are used in sewn bindings. The cloth holds the stitches and prevents the paper from perforating and tearing.