joint creases

A creased or scored line made from the outside of a cover from head to tail, parallel to the spine crease and a short distance from it towards the fore-edges of the cover. They are found on both sides of the cover and were made to assist the easy opening of a case-cover by sharing the hingeing movement required of a cover with the joint crease. A cover opened to 180° with only spine creases will make the cover material hinge through 180° along one narrow line, subjecting the cover to a great deal of stress. By adding joint creases, the movement is shared, with each crease only hingeing through 90°, thus reducing the strain on the cover.