oval-shaped and are relatively narrow, and twin-cylinder wings are more likely to be U-shaped and are wider. Some wings, known as bungee wings, incorporate elastic to constrain the wing when it is less than completely full and accelerate air dumping. These wings usually have elastic bands or cords wrapping the bladder area, but some designs use an elasticised shell or cords along a bellows fold gusset. Arguments for and against are: For: That constricting the wing improves streamlining and ease of gas dumping. Against: That the bungee will increase streamlining only if the wing is inappropriately large. That the bungee would forcefully expel gas from the wing if there is a small puncture. That the bungee creates a rough surface, increasing drag; however, in reality it is extremely unlikely that any increase in drag would be significant. That drag is a minor concern, but entanglement is a real hazard; for this reason, many technical and cave divers use "bungee-less" wings, as the elastic cords increase the risk of snagging. That the creases caused by the cords make it more difficult to dump the last of the air, and this may encourage divers to carry more weight than is strictly necessary, thus making good trim more difficult. Some manufacturers, such as OMS and Dive Rite make both and let the purchaser choose which style they prefer. The style which uses elastic only on the side gussets is more streamlined and smoother than the full wrap bungee style. Another variety, the dual bladder wing, contains a second, redundant bladder and inflation assembly, with the second bladder being intended for use if the primary bladder fails, either through a puncture, or through an inflation valve failing. Some technical divers may choose a dual bladder wing to have backup redundancy if the primary bladder fails. Detractors of this arrangement point out that if the extra bladder is inadvertently inflated, the diver may not realise that this has occurred, and it may result in an uncontrolled buoyant ascent, with a risk of fatal injury. This risk may be reduced by having only oral inflation for the backup wing, or requiring the same inflator hose to be used for both wings, and having the backup bladder inflation on the opposite side to the primary bladder. The wing may be a single or double skin unit. A single skin wing uses the same material for structure and