There are three main categories of belay devices: Tubular, Auto-Locking, and auto-blocking. In the past, belay plates, also known as stitch plates, were commonly used, and many varieties of plate-based devices that offered variable friction existed. At the moment these devices have all but left the market, leaving only the most successful designs.
Tubular: This type of device has one or two holes for a bight of rope (the curved section of rope) to pass through. The bight of rope is then secured to a climbing harness via a locking carabiner. The combination of the tubular device, carabiner, and belayer’s brake hand provide enough friction to stop a fall. The brake hand must be applied to stop the fall, meaning tubular belay devices do not lock on their own.
In a nutshell: These devices are lightweight, easy to use, and can be used with a single rope or double/twin ropes, depending on the model. They can also be used for rappelling, and they fit most sizes of climbing ropes. Most modern tubular belay devices incorporate a v‑shaped and ridged notch on the brake-hand side of the rope to increase friction and the ability to use smaller ropes. This is the most common style of belay device used for sport and traditional climbing. Almost all of these devices are made with 7075 aluminum, but a few companies have released steel versions, which are heavier and more durable.
Auto-blocking: Many of today’s devices have an additional attachment point that, through the use of an extra carabiner, allows the device to be used in an auto-blocking mode. Auto-blocking is different from auto-locking in that it only works for belaying from the top, as in a multi-pitch scenario. Originally popular with guides, these devices have become a mainstay for multipitch traditional climbers. The first iterations of these devices made lowering a stuck climber quite difficult, but the newer versions have solved this problem.
In a nutshell A tubular belay device with an auto-block feature is arguably the best all-around and most versatile belay device style, and every major manufacturer makes one. These devices can weigh as much as twelve ounces when made from many moving parts, although some are lighter than three ounces with a minimalist construction.
Auto-Locking (Self-Braking): These belay devices have camming mechanisms that lock down on rope under sudden forces, like a climber’s fall. Some of these devices use a spring and are fairly complex with moving parts, while others are made out a fixed piece of metal, using the carabiner as the camming mechanism. A bight of rope is placed into the device and then the device is secured to the belayer or anchor via a locking carabiner. These devices can be very helpful in arresting a climber’s fall and add another degree of safety since the device will catch a fall with no help from the belayer. However, an auto-locking device is not a substitute for good belay technique, as lowering a climber improperly is the most common mistake made with these devices.
In a nutshell: Auto-locking devices are most commonly used for sport and gym climbing, however, they can also be useful for aid-climbing during long and boring belays and rope ascension. If they are used in a traditional climbing scenario, care must be taken to ensure that any gear placed is bomber, as these devices offer a purely static belay. Since there is only one hole for a bight of rope, they are not usable for standard rappels, and are not recommended for use with wet or icy ropes.