Belaying refers to a variety of techniques used in climbing to exert friction on a climbing rope so that a falling climber does not fall very far [1]. A climbing partner typically applies the friction at the other end of the rope whenever the climber is not moving, removing the friction from the rope whenever the climber needs more rope in order to be able to continue climbing. The term belay is also used to mean the place where the belayer is anchored; this would typically be a ledge, but may instead be a hanging belay, where the belayer is suspended from protection in the rock. In a typical climbing situation, one end of the rope is fixed to the harness of the climber, using either a figure of eight loop, or a bowline or double bowline knot. The rope then passes through climbing protection which is fixed into the rock. This may be bolts which are permanently fixed into the rock, or it may be traditional protection, which is placed by the climber and then later removed without altering the rock. The top runs through the protection to a second person called the belayer. The belayer wears a harness to which a belay device is attached. The rope threads through the belay device, and by altering the position of the end of the rope, the belayer can vary the amount of friction which is applied to the rope. In one position the rope will run freely through the belay device and in another it can easily be held without moving because the amount of friction on the rope is so great. This is known as 'locking off' the rope.