The round sling insert fits a 1″ inside diameter tube with a 1-1/8" outside diameter. The black Nylon cap fits in the end of many sling rails. The sling rail is that part of an outdoor chair or chaise that holds the sling fabric. This is then bolted to the chair or chaise frame. The half oval sling insert is necessary to keep water out of the sling rails. Failure to do so can lead to furniture failure if water collects in the rail and it freezes. This causes the frame rail to swell allowing the sling to slip out of the rail. If this happens the chair or chaise will be ruined.