Speed bags are small targets used to increase hand speed, shoulder endurance, and rhythm. They are teardrop-shaped bags filled with a rubber bladder of air. Speed bags can function differently depending on the type of swivel to which they are attached, and each swivel offers certain advantages and disadvantages. There are three types: a ball and hook, chain-link, and d-pin. Each offers a different movement pattern. The ball and hook swivel provides an accurate and speedy rebound, but is considered very loud. The chain link swivel, on the other hand, is relatively quiet but can prove difficult to change bags. Finally, the d-pin swivel is easy changed, but afford sometimes awkward rebounds. Interestingly the speed bag is the only bag that boxers are not supposed to wear gloves when using it; however, because of the friction that it applied to a boxer's knuckles, it is advised that boxers wear wraps.