Open-weave cloth used to reinforce the hinges. It is pasted directly to the text block and boards of a book and then covered by the endpapers. Also called super or crash by bookbinders.
The first image shows a piece of unused mull at top and the front hinge of a book where the mull and tapes can be seen beneath the endpaper. The second example shows a book that became wet and has come apart. You can see the front board at left and the mull attached to the text block at right.