A parallel beam model is presented for the bending behavior of locally delaminated zones (creased zones) of paperboard. The bending moment and the force levels are strongly dependent on the geometry and elastic properties of the creased zone. A favorable condition for low crease stiffness is a large number of long interlaminar failures. Experimental measurements of the crease stiffness and maximum bending moment for paperboards with implanted defects of various lengths and numbers were in reasonably good agreement with the crease stiffness predicted by the parallel beam model. This is a rigorous analysis (with 29 equations) of the factors affecting the rigidity of cardboard when it is bent to form boxes. -- AATA