Bookbinding is the process of physically assembling a book of codex format from an ordered stack of paper sheets that are folded together into sections or sometimes left as a stack of individual sheets. The stack (signature) is then bound together along one edge by either sewing with thread through the folds or by a layer of flexible adhesive. Alternative methods of binding that are cheaper but less permanent include loose-leaf rings, individual screw posts or binding posts, twin loop spine coils, plastic spiral coils, and plastic spine combs. For protection, the bound stack is either wrapped in a flexible cover or attached to stiff boards. Finally, an attractive cover is adhered to the boards, including identifying information and decoration. Book artists or specialists in book decoration can also greatly enhance a book's content by creating book-like objects with artistic merit of exceptional quality.