There are two common types of monolithic protection: tapered wedges and hexes. Both are made specifically for climbing from lightweight aluminum. In use, both are wedged into cracks in the rock so that they are difficult to remove in one direction (usually down) and easy to remove in another (usually up). A tapered wedge, shown above, is a trapezoidal piece of aluminum (one to three centimeters across) attached to a loop of steel cable. A hex is a hexagonal tube of aluminum with a diameter roughly equal to its length, between one and six centimeters. A strong piece of cord is threaded through two pairs of little holes on opposite sides of the hex and tied into a loop. Monolithic protection is inexpensive (a typical piece is under $10), and, when carefully placed, strong (they can support at least 5 kN, about half a ton), but they cannot be used in all situations. For example, they do not work in cracks with nearly parallel sides.
An SLCD A spring-loaded camming device (SLCD) consists of a stem with an axle at one end holding four spiral-shaped spring-loaded cams. When placing an SLCD, the climber pulls a mechanism to retract the cams places it in a crack with the stem pointing down, and relases the mechanism, allowing the cams to spring back against the rock. When the SLCD is pulled downward (say, because of a fall), the spiral-shaped cams are forced harder against the rock, making it more secure. SLCDs are much easier to use than monolithic protection. They can adapt to the rock and hold themselves in place, making them usable in more situations. They have allowed climbers to climb many routes that were too dangerous to climb using other types of protection. The main disadvantage to SLCDs is cost: $50 to $100 each is typical. However, since each SLCD can adapt to a wider range of crack sizes than their monolithic counterparts, so only four or five sizes are needed. SLCDs also have the dangerous ability to ``walk.'' If not under tension, a SLCD can easily move in one direction, usually farther into a crack. This can make it difficult to remove, or more dangerously, move it to where it no longer holds. Monolithic protection usually doesn't do this, since it is usually firmly wedged into the rock.