slip sheet

n. A thin, smooth paper or other material inserted under a window mat to protect the mounted work. A leaf inserted between printed works to prevent the transfer of the image from the surface of one work to the back of an adjacent work. The process of inserting dummy pages in a work to indicate where separately printed illustrations are to be inserted.

Notes

Slip sheets3 often have figure numbers, titles, and other notes that aid in proper placement of the illustrations.