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. The blade type, voltage and amperage of a plug must match the blade type, voltage and amperage of a receptacle. For example: "L" (twist lock) plugs can only be used with "L" (twist lock) receptacles; "5" (120 V) plugs can only be used with "5" (120 V) receptacles; "20" (20 A) plugs can only be used with "20" (20 A) receptacles. Note: there are only 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.