The first electrical plug and socket were introduced in 1904 by Harvey Hubbell. Their introduction was caused by the development of labour-saving devices. However the cost of electricity for other purposes than lighting was higher, which led to low-power attachments.
Adapter plugs are considered to be simple in case they connect plug and socket without changing what passes through. To these belong garden hose and a serial port. Other adapter plugs are designed to change what is passing through, like transformer, which is used to adapt electrical current from high voltage to low voltage depending on the consuming capacity of electronics.
The voltage may be different depending on the industry standards and national regulations of a country. Many countries developed plugs of their own design without adopting the common standards. This can explain the existence of different styles of plugs. In most countries no single standard has been adopted, which creates additional problems for users. Plugs and sockets differ in size and shape, thus creating a demand for adapter plugs.
An American 2-pin plug can be inserted into a socket in one manner only. This type of plug is common in North America and the Caribbean. It is suitable for devices, which don't require a ground connection. However the types of sockets corresponding to such plugs haven't been used since 1964 and are found in old houses.
American 3-pin plug is common in the USA and Canada. Such plug has two parallel pins, like in American 2-pin plug, and a round ground pin, which is longer than parallel pins. To insert a grounded plug into an un ungrounded socket grounded adapter plugs are used.
European 2-pin plug, also known as Europlug, has two round pins. It is considered to be the most widely used plug, common in all European countries except Ireland and the United Kingdom. European 2-pin plug is symmetrical and hence can be inserted into the socket in any direction. The United Kingdom and Ireland have a different type of plug, which is used for electric razors. The sockets for such plugs are also suitable for Australian plugs. Australian 2/3-pin plug is used in Australia and New Zealand. Its two flat pins and an earthing pin form a V-shape. Such type of plug can be forced into American sockets, but it's not advised because of the differences in frequency and voltage. Australian 2/3-pin plugs are common in mainland China. Italian 3-pin plug is standardized in Ethiopia, Libya and Chile and in some parts of Spain. It includes two styles, which differ in spacing and diameter. Both of them are symmetrical and can be inserted in any direction.
The majority of European countries use the same two-pin plugs, with the difference in the way of earthing. Earth connection of European sockets is often unreliable, this concerns the old buildings with the earthed sockets installed on older electrical installations. Electric plug adapters are necessary in traveling, making an essential part of comfortable trip.