A belay device is a mechanical piece of climbing equipment used to control a rope during belaying.  It is designed to improve belay safety for the climber by allowing the belayer to manage their duties with minimal physical effort. With the right belay device, a small, weak climber can easily arrest the fall of a much heavier partner. Belay devices act as a friction brake, so that when a climber falls with any slack in the rope, the fall is brought to a stop.
Typically, when the rope is held outward, away from the body, it moves relatively freely, so the belayer can take up or pay out slack. When the rope is brought backward, to the side of the body, the rope is forced into tight bends and rubs against the device and/or against itself, allowing the belayer to arrest the descent of a climber in the case of a fall. This rubbing slows the rope, but also generates heat. Some types of belay devices can arrest a fall without the belayer taking any action, while others require the belayer to hold or pull the rope in a particular direction.
Belay devices usually attach to the harness of the belayer via a carabiner, and are usually made of aluminium or an alloy. Some belay devices can also be used as descenders for a controlled descent on a rope, that is abseiling or rappeling.
Many belay devices can be used to control either one rope, or two ropes in parallel. There are many reasons why the two-rope option might be chosen by a climber, including the consideration of reducing rope drag.
There are also auto-belay devices on the market which allow a climber to climb solo in their climbing gym.
This is a device that you feed a bight (loop) of rope through a hole or aperture and then hook it into a locking carabiner on the harness.
The Sticht plate was the first mechanical rope brake, named after its designer, Fritz Sticht. It consists of a small metal plate with a slot that allows a bight of rope to pass through to a locking carabiner and back out. This locking carabiner is clipped to the belayer who is then able to lock the rope at will.
Some plates had two slots for double ropes. The slots could also be different sizes for different diameter ropes e.g. 9mm and 11mm. A wide wire spring may be attached on one side to help keep the plate away from the brake carabiner to ease feeding and taking in rope. A smaller hole is often present for accessory cord to carry the device. Sticht plates are typically forged from aluminium alloy in a round disc shape, although other shapes such as rounded rectangles were also made.
Although any belaying plate with one or two slots is often called a Sticht plate, Fritz Sticht originally patented the design with Hermann Huber for Salewa GmbH in 1970, who sold it as the Salewa Sticht Bremse (Sticht Brake).
Sticht plates have become less popular since more modern designs provide smoother control over the rope and are less prone to jamming, especially when doubling as a descender.
This type of device generally has a tubular or rectangular shape. It is an evolution of the Sticht plate's concept by creating more surface area to dissipate heat and the ability to create sharper angles which creates a stronger degree of friction which has greater stopping power. As a result, this is generally the most common type of belay device used.
Besides arresting the fall of a climber, these devices can also be used for rappelling.
Sometimes just called an "eight", this device is most commonly used as a descender. A figure eight can be used for belaying, and indeed there are some which are designed specifically for belaying, however they are not generally popular due to the tendency to twist the rope. There are also variations on this design including DMM's "cardiac arrester" which does the same thing but is shaped like a heart. It is designed to help stop rope twisting. Figure eights, although not the most common belay device, are still frequently found in use. For most uses, a tubular style belay device is easier and safer to use. However, due to their tendency to twist rope and general disagreement about their safety, figure eights are often banned in climbing gyms.
Under the right conditions, assisted braking devices use a sudden load on the rope to engage a camming mechanism (known as active, or mechanically assisted braking devices) or pull the belay carabiner into a pinch point (known as passive, or geometrically assisted braking devices) to prevent the rope from passing through the belay device. The terms "self-" or "auto-locking" are discouraged, because it is recommended to always keep the brake hand on the rope,   there being conditions outside the correct function of braking. For example, icy, muddy, worn, or too-thin of a rope, and insufficient training and experience.
A guide plate, also known as an auto-blocking belay device,  is a metal plate with an elongated slot for the bight to go through and then a carabiner is attached so that when pull from the climber occurs the carabiner will be pulled to lock off the device.
A similar device to the traditional tubular belay device which has two extra loops; normally situated on the front and back of the device. When the device is attached directly to an anchor point with the use of a second carabiner through the larger of the two loops it performs a similar stopping function to that created with the guide plate. The device is also able to be used as a standard tubular device when belaying from the harness.
Statistically, by sales volume, the Grigri is the most popular mechanically assisted braking belay device.[ citation needed ] A Grigri, when properly used, assists in braking the rope with a camming device that clamps the rope in the event of a fall. Because of the braking mechanism, modified belay techniques are widely used, though Petzl, the device's manufacturer, has approved only certain techniques for instructing new belayers. Grigris reportedly give a harder catch than a regular belay device because they allow little to no rope slippage when catching a fall. On the upside, this is offset by the fact that the person being belayed falls a shorter distance. They are a proprietary design by Petzl.
The original Grigri is rated for 10 to 11 mm single ropes, while the newer, smaller Grigri 2 is rated for 8.9 to 11 mm single ropes and optimized for 9.4 to 10.3 mm ropes.  Trango sells a similar assisted braking belay device called the Cinch that is rated to work on ropes 9.4 to 11 mm.
Using a Grigri to bring up a second on a traditional anchor is however less favorable than other belay devices because the Grigri gives a more static catch with little to no rope slippage. This increases the amount of force exerted on the anchor which, in turn, increases the chance of anchor failure.
The Revo combines the easy and dynamic handling of a tuber with the backup by a centrifugal brake. Paying out and taking in rope works fluently without resistance. Only when the rope rushes with a faster speed than 4 metre/second through the device, it arrests within a few centimetres.
Self-belay devices are designed to allow solo climbing where the climber wears the belay device, or secures it to a fixed object on the ground. These devices automatically lock without any intervention when the rope passing through reaches a sufficient velocity (during a fall), but allow rope to move relatively freely whilst climbing.
Auto-belay devices belay climbers automatically, without needing a second person. While used by solo climbers outdoors, such devices are most commonly found hanging on or fixed to an artificially made climbing wall.
An auto-belay device may operate using friction, centrifugal force, hydraulics, or magnetic braking technology.  Auto-belay devices using magnetism as the main breaking mechanism are currently the most used, as they are self regulating. When a climber descends on a magnetic auto-belay, the speed of their descent is proportional to the climbers weight. 
A carabiner or karabiner, often shortened to biner, colloquially known as (climbing) clip, is a specialized type of shackle, a metal loop with a spring-loaded gate used to quickly and reversibly connect components, most notably in safety-critical systems. The word is a shortened form of Karabinerhaken, a German phrase for a "spring hook" used by a carbine rifleman, or carabinier, to attach his carabin to a belt or bandolier.
Figure-eight loop is a type of knot created by a loop on the bight. It is used in climbing and caving.
The Flemish loop or figure-eight loop is perhaps stronger than the loop knot. Neither of these knots is used at sea, as they are hard to untie. In hooking a tackle to any of the loops, if the loop is long enough it is better to arrange the rope as a cat's paw.
Climbing protection is any of a variety of devices employed to reduce risk and protect others while climbing rock and ice. It includes such items as nylon webbing and metal nuts, cams, bolts, and pitons.
A climbing harness is a device which allows a climber access to the safety of a rope. It is used in rock and ice climbing, abseiling, and lowering; this is in contrast to other activities requiring ropes for access or safety such as industrial rope work, construction, and rescue and recovery, which use safety harnesses instead.
This glossary of climbing terms is a list of definitions of terms and jargon related to rock climbing and mountaineering. The specific terms used can vary considerably between different English-speaking countries; many of the phrases described here are particular to the United States and the United Kingdom.
Rock-climbing equipment requires a range of specialized sports equipment, for training, for aid climbing, and for free climbing. Developments in rock-climbing equipment played an important role in the history of rock climbing, enabling climbers to ascend more difficult climbing routes safely, and materially improving the strength, conditioning, and ability of climbers.
Abseiling, also known as rappelling, is the controlled descent of a steep slope, such as a rock face, by moving down a rope. When abseiling, the person descending controls their own movement down the rope, in contrast to lowering off, in which the rope attached to the person descending is paid out by their belayer.
Belaying is a variety of techniques climbers use to create friction within a climbing system, particularly on a climbing rope, so that a falling climber does not fall very far. A climbing partner typically applies tension at the other end of the rope whenever the climber is not moving, and removes the tension from the rope whenever the climber needs more rope to continue climbing.
Lead climbing is a climbing style, predominantly used in rock climbing. In a roped party one climber has to take the lead while the other climbers follow. The lead climber wears a harness attached to a climbing rope, which in turn is connected to the other climbers below the lead climber. While ascending the route, the lead climber periodically connects the rope to protection equipment for safety in the event of a fall. This protection can consist of permanent bolts, to which the climber clips quickdraws, or removable protection such as nuts and cams. One of the climbers below the lead climber acts as a belayer. The belayer gives out rope while the lead climber ascends and also stops the rope when the lead climber falls or wants to rest.
The Munter hitch, also known as the Italian hitch, mezzo barcaiolo or the crossing hitch, is a simple adjustable knot, commonly used by climbers, cavers, and rescuers to control friction in a life-lining or belay system. To climbers, this hitch is also known as HMS, the abbreviation for the German term Halbmastwurfsicherung, meaning half clove hitch belay. This technique can be used with a special "pear-shaped" HMS locking carabiner, or any locking carabiner wide enough to take two turns of the rope.
An ascender is a device used for directly ascending a rope, or for facilitating protection with a fixed rope when climbing on very steep mountain terrain.
Petzl is a French manufacturer of climbing gear, caving gear, work-at-height equipment, and headlamps based in Crolles, France. The company was created by the cave explorer Fernand Petzl in the mid-1970s. Their three specialties are:
A Prusik is a friction hitch or knot used to attach a loop of cord around a rope, applied in climbing, canyoneering, mountaineering, caving, rope rescue, ziplining, and by arborists. The term Prusik is a name for both the loops of cord used to tie the hitch and the hitch itself, and the verb is "to prusik". More casually, the term is used for any friction hitch or device that can grab a rope. Due to the pronunciation, the word is often misspelled Prussik, Prussick, or Prussic.
Single-rope technique (SRT) is a set of methods used to descend and ascend on the same single rope. Single-rope technique is used in caving, potholing, rock climbing, canyoning, roped access for building maintenance and by arborists for tree climbing, although to avoid confusion in the tree climbing community, many have taken to calling it "stationary" rope technique.
Rope-solo climbing or rope-soloing is a form of solo climbing, but unlike with free solo climbing, which is also performed alone and with no climbing protection whatsoever, the rope-solo climber uses a complex self-belay device and rope system to protect themselves in the event of a fall.
A Reverso is a belay device developed and patented by Petzl, used for example in rock-climbing and other activities which involves rope-work. Another version of this device is the Reversino, intended for use with thinner ropes.
A dynamic rope is a specially constructed, somewhat elastic rope used primarily in rock climbing, ice climbing, and mountaineering. This elasticity, or stretch, is the property that makes the rope dynamic—in contrast to a static rope that has only slight elongation under load. Greater elasticity allows a dynamic rope to more slowly absorb the energy of a sudden load, such from arresting a climber's fall, by reducing the peak force on the rope and thus the probability of the rope's catastrophic failure. A kernmantle rope is the most common type of dynamic rope now used. Since 1945, nylon has, because of its superior durability and strength, replaced all natural materials in climbing rope.
A Grigri is an assisted braking belay device manufactured by Petzl designed to help secure rock-climbing, rappelling, and rope-acrobatic activities. Its main characteristic is a clutch that assists in braking under a shock load. The success of this device has led to grigri becoming a common name for devices of this type. In 2011 a new version, the Grigri 2, was released to replace the original 1991 model. Petzl released the Grigri+ in 2017, adding safety features to the original design, and 2019 saw the release of an updated version of the device, simply called the Grigri. It is named for the African amulet gris-gris, believed to protect the wearer from evil.
A Petzl Stop is a descender used primarily in caving and formerly used for industrial rope access made by the French company Petzl.
The Revo is a semi-automatic belay device manufactured under the brand Wild Country by manufacturer Oberalp, for sports climbing with single ropes. It appeared on the market in autumn 2018.
hence the proper phrase used for these devices—"assisted braking" rather than "self-locking."
There aren't any auto-lockng belay devices outside of climbing walls because the "official" term is assisted locking braking device