Abstract: A window construction (20) includes a support frame (22) having a front channel member (52) for receiving a slidable window sash (26), and a rear channel (56) for receiving a fixed window sash (24). Front and rear channels (54) and (56) are divided by a central flange (52). Weatherstrip means (136) are mounted on sashes (24) and (26) to project transversely outwardly from the sashes to press against the adjacent face of support frame central flange (52) and against an adjacent surface of the opposite sash to thereby form an unbroken seal around the entire perimeter of each sash. The support frame 22 also includes a compound shaped mounting flange (32) having a diagonal section (76) extending outwardly and diagonally forwardly from front channel (54), and a perimeter section (78) extending outwardly from the outer edge of the diagonal section (76) to lie in a plane forwardly offset from and substantially parallel to the plane defined by sashes (24) and (26).