Apparatus for measuring the stiffness of creases of a packet blank comprises first and second spaced apart guides, on which a flat blank may be supported, and a plunger which is movable between the guides to deform the blank about a crease(s). The plunger brings part of the blank into contact with a bearing surface, and a panel(s) bent relatively of that part rests against force measuring means. The force measuring means provides a signal indicative of the resistance to bending of a crease about which the panel(s) has been bent.