The most widespread system in use today is the "three-man system", that uses one referee and two linesmen. Another less commonly used system is the two referee and one linesman system. This system is very close to the regular three-man system except for a few procedure changes. With the first being the National Hockey League, a number of leagues have started to implement the "four-official system", where an additional referee is added to aid in the calling of penalties normally difficult to assess by one single referee. The system is now 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.