Andersen&reg; 939 Narroline Sash Balancer Match your windows (lower sash) visible glass size (width & height) to the chart Works with Perma-Shield Narroline Double-Hung Windows manufactured from 1968 to Present. Sash balancers are metal boxes that contain a nylon cord under tension. The balancer number is a code for the tension and the cord length. If you replace one balancer you should replace both balancers.