Abstract: A window assembly including two horizontally slidable window units normally occupying separate portions of a window opening, which window units are selectively movable across the opening to provide a ventilation to an interior space. Each window unit comprises a four-sided rectangular frame and a windowpane haivng a hinged connection along a side edge of the frame. The windowpane can be swung out from the frame to a position normal to the frame, whereby the outer surface of the windowpane can be mannually cleaned from within the building space.