The AG-13 Air Gap Fitting is used for indoor installations of all 4" - 6" Model 475ST/475STV Reduced Pressure Principle Backflow Preventers for the connection of a drain pipe to the backflow preventer, directing nuisance water discharge to a safe drain. The AG Air Gap Fitting can be mounted to the body of any drain line made of any standard piping material. The AG prevents a cross connection between the drain and potable water supply.