Plates are illustrations that are printed separately from the text of a book.
Many plates are smaller than the text block page and are attached to a page of a text block by tipping them in along one edge.
This is common even in books with sewn text blocks because plates are often printed on coated paper that is heavier than the paper used to print the text block. When the original glue dries up, the plate separates from the text and falls out of the book.
The best way to tip a plate back into a book is to apply glue to the edge of a plate using the second technique outlined in the previous post – Reattaching a Text Page.
It can sometimes be a better choice to use the Hinging-In an Illustration or Plate outlined in a separate post.