The spine of a booklet is the side/edge where it is bound. For perfect bound booklets, the width of the spine varies based on total number of inside sheets and how thick each of the inside sheet is.
It is important to know the spine width of your perfect bound booklet so you can design the cover properly. As the cover of a perfect bound booklet wraps around the inside pages, the spine width needs to be added to the width of the covers (front and back).
To calculate the spine width you can use our perfect bound booklet maker tool.