What are gutters?
Because perfect bound books cannot lay completely flat without breaking, there is a portion of each inside page that will lose visibility into the spine. This inside gutter can take up between ¼” and 3/8” of each page. Be mindful of this while designing and add an extra ½” cushion to the binding-side margins of each page of your perfect bound book. For more about setting up your margin and safety lines, click here.
Note: If you are adding page numbers to your project, then do not put them on the inside of the pages close to the spine. You will risk losing visibility.