Joint protectors are made to serve 2 main purposes: to reduce the risk of bending joint pin, and to protect the joint collars from damage due to an impact. 5/16-18 is a very common joint style used by Sterling, Viking, and more. These are plastic joint protectors, and is a set of two: one for the pool cue shaft, and the other is for the pool cue butt. Common cue brands featuring the 5/16 x 18 joint style are Meucci, Sterling, Action, Stealth, Viking, and Mali.