Product details:

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. - The proper balancer must be used to maintain the lower and upper sash in an open position. - If you replace one balancer you should replace both balancers.