Ring gags are similar in concept to a ball gag but replace the ball with a metal ring which forces the captive's mouth 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 (such as a penis). This makes ring gags particularly effective in scenes that have a strong D/S element.
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 leather or rubber - metal alone can chip or damage a captive's teeth.