Descenders, or rappel devices, are designed to apply friction to a rope in order to control the rate of descent. They are attached to the caver through the master attachment point, which is usually a demi-rond maillon or Petzl Omni on a caving-style harness. Types of rappel devices include knots such as the Munter (or Italian) hitch, figure eights, tube devices, brake bar racks and bobbins. The general idea is that the rope takes a tortuous path through the descender and the friction that is generated helps to control speed, and ultimately this energy is dissipated as heat. The devices favored by cavers are the brake bar rack and bobbin. The most common forms of the brake bar rack are the 6-bar J-frame rack, originally developed by John Cole in the 1960’s. The most common versions of the bobbin are the Petzl Simple and Stop, which were originally developed by Dressler, also in the 1960’s. The advantages of these devices for caving are that they allow a wide range of friction control for varying conditions underground, they allow easy installation and removal from the rope without disconnecting them from the harness, they don’t cause a twist in the rope, and they are easy to lock-off for hands-free operations such as rigging, or mid-rope maneuvers such as changeovers, crossing knots, or crossing rebelays.