Like case-bound books, perfect-bound books are also made up of stacked signatures. These are gathered into a book and the edges of the spine are ground off (or notched). When this book block is glued into a paper cover, the glue that attaches the signatures to the spine can flow into the notches or ground-off areas. The increased surface area for the glue allows for more permanent adhesion. The covers and book blocks are then trimmed flush. Unlike case binding, perfect binding involves only gluing the spine to the cover. Without reinforced endsheets or a binders board cover material, perfect-bound books are less durable than case bound books but are significantly less expensive. Sewing the signatures and/or notching the spine rather than grinding it improves durability.