Abstract: A mounting frame is adapted to be fixed on one side and is formed with a cavity which is open on the opposite side of said mounting frame. The mounting frame comprises an oblique annular peripheral sealing lip, which protrudes on said opposite side and surrounds said cavity and opposite to said cavity defines an undercut annular peripheral groove. Multi-pivot hinge means are accommodated in said cavity and secured on one side to said mounting frame. A sash is secured to said hinge means on the other side thereof and is movable relative to said mounting frame to a closed position and arranged to sealingly contact said sealing lip in said closed position. Said sash has an annular peripheral channel which is open toward said mounting frame and arranged to receive said sealing lip in said closed position.