assess penalties for violations of the rules. When a penalty is being assessed the referee will stand at center ice and announce the penalty to the entire arena. However, the linesmen can report a variety of penalties, such as Too many men on the ice and major penalties, to the referee, who may then assess the penalty. The referee also conducts the opening face-off in each period and face-offs that follow after a goal is scored, which are done at the center ice face-off dot. If a goal is challenged, the referee or referees are the ones who review the play. Linesmen Linesmen or linespersons are primarily responsible for watching for violations involving the centre line and the blue line. Such infractions include icing and offside, after which the linesmen conduct faceoffs. They are also expected to break up scuffles, fistfights and other altercations that occur during the game. In some leagues, the rules allow linesmen to call some penalties (such as Too many men on the ice), while others only allow them to report the infraction to the referee. Assistant referees In some leagues, the linesmen are given the title of the assistant referee. When given this title, they are given more responsibility to stop play and to call penalties that the referee may not see. The NCAA previously used this designation prior to altering its officiating systems in 2006. On-ice officiating systems The three-official system uses one referee and two linesmen. This is the most commonly used system. The NHL previously used this system until changing to the four-official system in the late 1990s. The highest level for this system is the ECHL, which does official development for the NHL. The four-official system adds a second referee for a total of two referees and two linesmen. In this system, each referee works either the lead/front or the trail/back position. The lead position is normally in the corner of the end zone, while the trail position is in the neutral zone, diagonally opposite the lead referee. As the game transitions from one end of the ice to the other, the lead becomes the trail and vice versa. The four-official system is used in the NHL, AHL, and at other high levels, such as major junior, junior 'A', and top European leagues. In 2007-2008, college hockey used the system in some games on a trial basis. The IIHF first implemented