Knapp's design was obsoleted in the U.S. before the modern NEMA designations, but is still used in some other countries including China, Argentina and Australia. The configuration of the socket was not operable with existing two-contact unearthed plugs. Other earthed sockets that are widely used in the U.S. today are operable with unearthed plugs. It is sometimes claimed (e.g., in Illumin) that the modern American version of the earthed plug, was invented by Philip F. Labre who was issued a U.S. patent for an earthed socket and plug in 1928. However, Labre's design is no more similar to the modern version than Knapp's earlier design. The German Schuko-system plug is believed to date from 1925 and is attributed to Albert Büttner. As the need for safer installations became apparent, earthed three-contact systems were made mandatory in most industrial countries. During the first fifty years of commercial use of electric power, standards developed rapidly based on growing experience. Technical, safety, and economic factors influenced the development of all wiring devices and numerous varieties were invented. After the two-prong electric plug was introduced in the 1920s, the three-pin outlet was developed. This format was introduced in order to mitigate the effect of a short circuit event, as the supply would be neutralised with earth. Gradually the desire for trade eliminated some standards that had been used in only a few countries. Former colonies may retain the standards of the colonising country. Sometimes offshore industrial plants or overseas military bases use the wiring practices of their controlling country instead of the surrounding region. Some countries have multiple voltages, frequencies and plug designs in use, which can create inconvenience and safety hazards. Hotels and airports may maintain sockets of foreign standards for the convenience of travellers. By 2018, there were 15 plug and socket types around the world. "De facto" standards became formalised as official national and international standards. The earliest is believed to be British Standard 73 "Wall plugs and sockets (five ampere two-pin without earthing connection)" which was first published in 1915. The International Electrotechnical Commission in 1934 established technical committee TC 23 for electrical fittings. Only two meetings were held before the outbreak of the Second World War. In mainland Europe, since 1951 the International Commission on the Rules for the Approval of Electrical Equipment (CEE) has published a standard (CEE 7 "Specification for Plugs and Socket-Outlets for Domestic and Similar Purposes") describing the plugs and sockets used.