Ice hockey
Officials

The most widespread system is the "three-man system", which uses one referee and two linesmen. A less commonly used system is the two referee and one linesman system. This system is close to the regular three-man system except for a few procedure changes. Beginning with the National Hockey League, a number of leagues have implemented the "four-official system", where an additional referee is added to aid in the calling of penalties normally difficult to assess by one referee. The system is used in every NHL game since 2001, at IIHF World Championships, the Olympics and in many professional and high-level amateur leagues in North America and Europe.
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