21" Window Channel Balance; 2020 or 20B 10, 20, 30, 40, and 50 Weight Range Balances in Lengths from 13&quot; to 43&quot; (330 mm to 1.09 m) Measures 5/8&quot; (16 mm) Depth, but Referred to as 1/2&quot; (13 mm) in the Trade