Abstract: Disclosed herein is an improved window opening structure for a window assembly having an outer frame defining an inner opening having parallel vertical edge surfaces; at least one window sash having vertical side jamb rails, a top rail extending between said jamb rails and a bottom rail extending between said jamb rails; and sash being mounted in said outer frame for sliding movement between a lower window-closed position and an upper window-opening. The opening structure includes a U-shaped opening means having a pair of lift legs and a central lift rail portion extending therebetween and means fastening inner end portions of said lift legs to each of said jamb rails.