A ring gag is similar in concept to a ball gag but the ball is replaced with a metal O-ring which forces the wearer's mouth to stay open. In BDSM play, the main reason for doing this is to make it impossible for the captive to refuse to accept anything place in his/her mouth. Conversely, they are ineffective in silencing the wearer, though coherent speech is prevented.
Ring gags come in various dimensions and sizes, and most are measured according to the inner diameter, not the actual size of the ring itself. Most come wrapped in rubber - metal alone can chip or damage a captive's teeth.