Often used for publications such as cookbooks, textbooks, and workbooks, comb binding allows a book to be opened flat. Rectangular holes are punched in the pages of the book, and then the teeth of the plastic comb are pushed through the holes. Because the combs are coil-like and curly, the teeth curve back under a spine-like collar that forms a solid spine for the bound book (Figure 3.25). The plastic combs come in a variety of colors and diameters. Comb-bound books usually use heavier stock for the front and back covers, or they use clear plastic sheets as a protective first page.