A sliding window assembly including a window frame and at least three panes disposed within the window frame in offset vertical planes. The panes are arranged for sliding movement together upwardly within the window frame or together downwardly within the window frame. One or more of the panes can be fixed. The panes have a fully open position and a fully closed position. In the closed position, respective edges of the panes overlap while in the open position, the entire panes overlap. The panes can be connected to a counterweight arrangement by a pulley arrangement, which includes a vertically movable balancing weight that is equal to the weight of the panes, or the panes can be connected together to counterbalance each other.