The National Electrical Manufacturer's Association (NEMA) has developed standards for input (plugs) and output (receptacles) to ensure that end users do not accidentally connect devices to power sources with incompatible voltages and amperages--which could create dangerous conditions. Users should be familiarized with NEMA's plug and receptacle naming conventions, so they can match the correct plug with the correct receptacle. The blade type, voltage and amperage of a plug must match the blade type, voltage, and amperage of a receptacle. If it does not match, it will be physically impossible to connect the plug to the receptacle, since NEMA's standards ensure the plug blades are shaped and spaced differently. For example: "L" (twist lock) plugs can only be used with "L" (twist lock) receptacles; type "5" (120 V) plugs can only be used with type "5" (120 V) receptacles; type "20" (20 A) plugs can only be used with type "20" (20 A) receptacles. Note: there are two receptacles (5-15/20R and 6-15/20R) that are designed to accept both 15- and 20-amp plugs. 5-15/20R receptacles accept both 5-15P and 5-20P plugs. 6-15/20R receptacles accept both 6-15P and 6-20P plugs. There are no other exceptions. For 3-phase plugs and receptacles "P" refers to "power" (or "hot"), "N" refers to "neutral" and "E" refers to "earth" (or "ground").