Stretchy silicone band with hard plastic ball attached Slips over any dive cylinder from 6 inch diameter and up Pull and release the ball to create a reverberating 'clang&'; on your tank while underwater Plastic ball is a high visibility, neon yellow color to assist with quick location at depth Essential safety equipment: never dive without a signaling device This Quality Tank Banger is great for getting the attention of your buddy, your students, or other divers from long or short distances. It is high visibility yellow in color for easy use.