A vacuum breaker is an attachment commonly placed on a bibcock valve, toilet, or urinal flush valve, that prevents water from being siphoned backward into the public water system. This prevents contamination should the public water system's pressure drop. The backflow preventer is a more complex valve that accomplishes much the same purpose. A check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid (liquid or gas) to flow through it in only one direction. Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave.