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 called signatures or sometimes left as a stack of individual sheets. Several signatures are then bound together along one edge with a thick needle and sturdy thread. 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.