The black, one and one quarter inch, multi-gauge ball insert is a cap designed to fit inside a chair or chaise frame tube. It creates a smooth, half round, decorative cap for the end of the tube. It's decorative but it keeps water out of the furniture rails that can lead to rust as well as broken frames due to the water freezing and expanding.
The Multi-Gauge Ball Insert Cap fits a frame tube size with a 1 and one quarter (1-1/4") inch outside diameter. This would be for a 14 to 20 gauge tube