A belay device is used in belaying, when one climber secures the rope for another climber as he ascends. It's the basis of climbing safety -- it holds the rope and the other climber in case of a fall. The rope links the climber and the belayer in a safety partnership, allowing the climber to fall without fear of hitting the ground. It's also used in rappelling, or sliding down the rope to reach the ground.
Sometimes called a BD, a belaying device allows the belaying climber to hold the active climber's fall via the friction of the rope as it runs through the device. The rope, anchored to safety gear at the top of the wall, is attached to the climber and the belayer. The climber is tied to the end of the rope, while the belayer is attached to the rope with a belay device, a mechanical device that makes it possible for the belayer to hold the climber's weight. The belayer must keep the rope snug on the climber as he ascends, hold him if he falls, and lower him back to the base after reaching the top of the wall.
Belay devices are available in many different sizes, styles and shapes. Some are interlocking devices. Made of aluminum or an alloy, BDs are an essential piece of gear for every climber to own and use.
The belay device attaches to the harness of a belayer using a carabiner, a metal loop with a spring or screwed gate. Carabiners also come in different shapes, including the versatile oval, the D shape with a larger opening that makes clipping the rope easier, and locking versions that are ideal for belaying or setting anchor. Good quality carabiners can help your rope to last longer.