Every climber should have his or her own belay/rappel device. Important factors to consider in selecting these devices include braking power, ease of rope handling, versatility, and security.
Most belay/rappel devices work by creating a bend (or bight) in the rope that produces a manageable level of friction. A bight of rope is pushed through an opening slot and is then clipped into a locking carabiner. Braking and holding power is achieved by the choking action of the device pulled against the locking carabiner.
The devices come shaped like plates or tubes. They work for both belaying and rappelling with either double or single ropes. There are also autolocking devices, which clamp the rope automatically when loaded. The Petzl Gri Gri is the only autolocking device discussed in this book. It is much heavier and more expensive than plate or tube style devices, but it does work beautifully. It can only be used for single-rope belaying or rappelling.
Figure-eight devices are for rappelling with either double or single ropes. They offer smooth rappelling but can twist the ropes. Figure eights are limited in their use as a belay device. Check the manufacturer’s recommendations.