needs to be used on land. However, the standard bent sheet-metal backplate is ergonomically unsuited to this function. Backplate The backplate is usually made from a single piece of stainless steel or anodised aluminium, bent along four lines to form a channel running vertically down the center. The plate is about 15” (38 cm) long and 10” (25 cm) wide. There are two slots near each of the shoulders and hips, where the harness passes through, and another slot at the bottom of the plate where the crotch strap attaches. There is also one or more pairs of holes in the channel to be used for cylinder attachment; the holes of each pair are usually 11” apart. A variation on this design uses another two parallel bends to form a shallow trough down the back of the central channel, which stabilises a single cylinder strapped to the centreline. Steel backplates are commonly used when the buoyancy of the diver's other equipment (primarily cylinders and exposure protection) would need a weightbelt, as the negative buoyancy of the steel plate can replace some of this weight. Aluminium backplates are commonly used when the diver would not require a weightbelt (such as when wearing heavy steel cylinders) or when the mass of the backplate must be kept low for air travel. Backplates are occasionally made from other materials, including carbon fiber reinforced plastics, titanium, and ABS plastic. Lightweight versions of the backplate are available with inessential areas cut away to reduce the weight. When taken to the extreme, a single cylinder is used as the longitudinal stiffener, and the backplate is reduced to a skeletal frame at the level of the waist belt, which stabilizes the cylinder against rolling on the diver's back. Wing The wing is an inflatable buoyancy bladder, similar to that in other varieties of BCD, except that it is not in or permanently attached to the harness part of a BCD. As with other BCDs, wings have an inflation valve on a corrugated hose, dump valve, and over pressure valve. Wings are usually oval (annular, doughnut or toroidal) or U-shaped (horseshoe), and are designed to wrap slightly around the diving cylinder(s) when inflated. Wings are usually designed to be used with either a single diving cylinder or twin cylinders, although some manufacturers make wings that they recommend for both single and twin cylinder diving. Single-cylinder wings are most commonly