A belay/rappel device is a metal clamp that cinches on the
rope. This braking action lets hold a fellow climber on rope tension, or rappel. Rappel/belay devices are as plentiful in design as fishing lures.
On the whole, though, there are two basic designs, the “tube,” (few are actually tube shaped these days, but years ago most were and the name stuck),
and “brake assist” types.
The tube is the most economical, lightest and versatile, letting you belay and rappel on single and double ropes, an advantage for multi-pitch routes with
rappel descents. Some tube designs are partly self- locking, offering a brake-assist without any moving parts—nice. Other designs might have
only one slot, for a single rope.
Disadvantage? A tube can produce so much rope friction it can be difficult for a lightweight person to rappel or be lowered, or a tube might not generate
enough friction for a heavier-weight climber. Rope diameter and the size of the slot(s) in the tube will determine friction.
Brake-assist devices have an internal cam or other feature that pinches the rope when it is weighted. These are popular among sport and gym climbers, who
might catch dozens of falls for hours at a time on a single route. Brake-assist devices are more expensive, heavier, and more complicated to use than
the simpler tube. Most also require a steeper learning curve.
Check out Rock and Ice's belay and rappel device reviews.