The black one inch multi-gauge ball insert is an end cap designed to fit a chair or chaise frame tube with a smooth, half round, finished cap. It is decorative but it also keeps water out of the furniture rails. Trapped water can lead to rust as well as broken frames due to the water freezing and expanding.
This black Multi-Gauge Ball Insert Cap fits a frame tube size with a 1 inch outside diameter. This would be for a 14 to 20 gauge tube.