A spline is the long rounded flexible piece of material you will see in the rails of your sling patio or pool furniture awnings or slide out covers on RVs which insert in the loop sewn on each side of the fabric sling. This is used to keep the fabric securely attached across the two rails. When cut spline is called and sling rod and is inserted into the sling pocket to keep the sling from pulling out of the sling rail. The spline must be large enough to keep from slipping through the opening slot running along the rails yet not too big to fit well in the ends of the rails for easier installation. Most quality patio or pool sling furniture such as Winston Woodard Tropitone Mallin Carter Grandle Brown Jordan Agio Innova Telescope Lyon Shaw are designed to use spline material to install the fabric replacement slings on the frame. There are some exceptions like Homecrest or Samsonite which use two layers of material on some of their furniture to slide the slings over an internal frame. If you do not have your old slings you may measure the slot width running up the rail and find which size you will need. Make sure you find the widest area of the rail slot and be sure your spline choice is just slightly bigger than that rail slot area so that it will keep the fabric in place when tightened.