Abstract: A pair of sliding window sashes have overlapping ends, each of which includes a vertical sash rail to which a vertical metal bar is joined between the rail and the other sash. Each bar has a pair of vertical slots in it separated by a vertical flange and facing the opposite end of the sash that carries the bar with the outer side wall of the outer slot forming a tongue extending into the outer slot in the other bar to interlock the two bars while the window is closed. The inner side wall of each groove extends outwardly beyond that groove toward the opposite end of the sash to form a vertical flange. A vertical sealing weather strip is connected to the side of each bar opposite its slots and engages the flange on the other bar to form a seal.