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A patent classification system is used by a patent office to list patent documents according to the similar technical features of the inventions. The same document may be listed in several classes. The International Patent Classification (IPC) is used internationally. The United States Patent Classification (USPC) is defined by the U.S. Patent and Trademark Office. The European Classification (ECLA) is based on the IPC, but adapted by the European Patent Office to meet its own requirements. The Derwent classification system is defined by an enterprise.

There are over 400 classes in the U.S. Patent Classification System, each having a title descriptive of its subject matter and each identified by a class number. Each class is subdivided into subclasses. Each subclass also bears a descriptive title and is identified by a subclass number. The subclass number may be an integral number, or contain a decimal portion and/or alpha characters. A complete identification of a subclass requires the class and subclass number and any alpha or decimal designations, e.g., 414/171.2A identifies Class 414, Subclass 171.2A.

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