So far, there is no unified world-wide standard for plug and socket size. Every country has its own size standards system.
There are mainly three systems, differentiated by types sizes:
-Round type System
-Flat type System
-Rectangle type system
The round type system is used for most continental European countries: such as Germany, France, Austria, Belgium, Russia and so on.
Rectangle type system is used for most of the original Commonwealth countries: the United Kingdom, India, Pakistan, Malaysia, Singapore and Hong Kong.
Flat type system is used mostly for the United States, Japan, Australia, New Zealand, China and so on.