Sling spline is a rod of thick nylon designed to be inserted into an outdoor furniture sling rod pocket to keep the sling from pulling out of the sling rail. Cloth slings are inserted into both side of a sling chair back or bottom sling rail. This rail is bolted to the chair or chaise frame and pulled tight to stretch the sling fabric. The slings are held tightly in the frame rail because the sling spline is too large to be pulled, along with the sling, out through the gap in the sling rail.
Warning: if the sling rail has been damaged or spread apart due to freezing water in the sling rail, using a bigger sling spline will not render the chair safe again.