The 4-Sided Exterior Professional by Security Boss Manufacturing LLC uses a heavy duty stainless hinge connection for the flap. This is a great feature for kennels that may have many different breeds that use the kennel runs. The hinge connection design is more forgiving when taller dogs are using the openings as their backs tend to touch the top of the opening. In doors without this hinge feature, flaps may wear prematurely if pressure is applied repeatedly at the flap connection point. The hinge also features a weather seal that helps eliminate drafts through the hinge mechanism.