The spring belt consists of 3 stay bars (one for the front, one for the middle, and one for the back of the book) that are linked together with a continuous spring that is sized to fit the specific book dimensions (page height and thickness.)
After extensive use the spring belt may become stretched and will no longer hold the markers in place. Replacing the spring belt will extend the life of your markers.
(Marker sets come with a spring belt. Purchase this if you need to replace your belt).