What is a backflow prevention assembly?

A backflow prevention assembly is a means or mechanism to prevent backflow.  The basic means for preventing backflow is an air gap, which either eliminates a cross-connection or provides a barrier from backflow.  The basic mechanism for preventing backflow is a mechanical backflow preventer, which provides a physical barrier to backflow.  The principal types of backflow preventers are the reduced-pressure principle assembly, the pressure vacuum breaker assembly and the double check valve assembly.