Gutter

The gutter refers to the inner margin area where the pages join the spine. It’s the space on each page next to the binding, shaped by the curve when the book opens. Its size varies. Thicker books or glued bindings need a wider gutter (0.75-1 inch) since they don’t open flat easily, while thinner or sewn books can use a narrower one (0.5-0.75 inches) because they lay open better.