Abstract: The internal muntin bars of an insulated window assembly include spaced apart component bars. Each component bar extends between opposite sides of the perimeter bar and has cooperating notches for engagement with the adjacent component bar which crosses it. A telescoping fastener with disc-like heads is received in a center opening at the intersection of crossing muntin bars and serves to lock the bars together and fill the space between the glass panes. The legs of a U-shaped retainer are received in the open ends of the component bars and are secured to the perimeter bar.