Comb binding (sometimes referred to as "cerlox" or "surelox" binding) is one of many ways to bind pages together into a book. This method uses round plastic spines with 19 rings (for US Letter size) or 21 rings (for A4 size) and a hole puncher that makes rectangular holes. Comb binding is sometimes referred to as plastic comb binding or spiral comb binding.
To bind a document, the user first punches holes in the paper with a specialized hole punch. Pages must be punched a few at a time with most of these machines. If hard covers are desired, they must be punched as well. In bulk applications, a paper drilling machine may be used.
Then the user chooses a spine size that will match the document. Standard sizes are NaNinches (for 16 sheets of 20# paper) up to 2inches (for 425 sheets). Spine lengths are generally 11inches to match the length of letter-size paper.
The rings on the spine open and insert into the holes in the page, then rest against the body of the spine, resulting in a closure that can be opened again for making changes to the book.
With this bind, the book lies flat but cannot be opened 360 degrees. For a book that can be opened such that the covers touch, a spine that does not have an obstructive body, such as a coil binding, is a better option.