In 1978, the Patent Cooperation Treaty (PCT) went into effect to streamline multi-country patent applications. Published patent applications filed through the PCT (as opposed to through an individual country) have the prefix WO. As patents get issued in individual countries, additional patent numbers, with country prefixes, will be added to the "patent family", and certain databases will link all of these together by the PCT Application Number.
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There are currently two classification systems in use: the International Patent Classification (IPC) and the new Cooperative Patent Classification (CPC). The CPC replaces both the USPC (United States Patent Classification System) and the ECLA (European Classification System). The CPC replaced the ECLA in 2013, and the USPC in 2015.