a door frame with a substantially continuous sealing surface supports a door with a substantially continuous sealing member mounted thereon . a stepped u - shaped frame member has an opening at one end that is closed with a square - cut member , such as a threshold . the frame , threshold and hinges mounted on the frame provide a substantially continuous sealing surface . the bores of the hinge knuckles may be matched with bushings in the bores , so that the bushings must be properly oriented . the heads of the bushings may have at least two steps or raised contact surfaces and at least two indentations or depressed contact surfaces . when the doors are opened , the steps on their bushings in the pivoting hinge leaves drop into the indentations in the adjacent fixed hinge leaf and hold the door in position .