Socket sizes are not universal and as of June 2015, there were 15 types of sockets in use as recognized by the U.S. Department of Commerce International Trade Administration. The socket types differ in size, voltage capacity, frequency and terminal types.
Sockets are assigned arbitrary letters starting from letter A through to letter O. Some sockets are compatible with more than one plug type. The U.S. Department of Commerce International Trade Administration reports that plugs type A and C are the most frequently used in the world.
The United States, Canada, Mexico and Japan are examples of countries that use plug types A and B. Socket type B is compatible with both A and B plugs.