Turning our attention to the window sash itself: The window consists of a sandwich of an inner and an outer aluminum frame used to clamp the two panes in a double channel gasket. The black pieces visible in the middle are Bakelite spacers which provide an equal distance ensuring an uniform clamping force around the window frame. The top frame is the outer frame, the square one on the bottom the inner which includes the lip used to attach the entire assembly to the window opening.

Photo courtesy of Jon Snyder