Abstract: A rectangular multi-apertured shadow mask electrode having a rectangular peripheral reinforcing frame of L shaped cross section is mounted within the rectangular envelope of a color cathode ray tube adjacent to the faceplate with a spacing gap between the frame and the side wall of the envelope. An electron shield of thin metal (e.g. Al) foil extends around and across the spacing gap to prevent scattering of beam electrons during overscan of the mask. The shield comprises an inner flange attached to one flange of the frame by means of tabs and an outer flange pressed against the side wall of the envelope. At each corner, the shield includes a portion of U-shaped cross-section disposed within the spacing gap with the sides of the U resiliently engaging the envelope side wall and the other flange of the frame.