The Leopold Mouth Gags do one thing. quite simply they work! Traditional gags continuously slip out and pressure causes them to close and run the risk of damaging the canine teeth. The soft rubber jaws eliminate the risk by engaging a number of teeth for nonslip performance. Thumb screws allow the mouth to be positioned and then the gag is locked into place. Works on animals with or without canine teeth Made entirely in the US of US materials. Lifetime warranty of all parts (repair or replacement) with the exception of the rubber pads for a minimum of cost. Medium is recommended for Dogs 20lbs-30lbs.