A Jon line is a short line used to fasten divers to something. The line is 10 feet long and equipped with a clip at one end. These clip is fastened to the diver's harness, and the other end is used to fasten the line to an anchor line. In a current, this relieves the diver from having to holding on to the anchor line during their decompression stops. It can also be used to tether your equipment to the dive boat before or after the dive. This enables the diver to get into or out of his or her equipment while in the water. A real must have.