and nine line umpires. Each line umpire is assigned to one line or, in the case of a short-handed crew, a position in a system. For example, a line umpire on the receiver's side may have to cover the center service line then, following the service, move to one of the sidelines. The line umpire signals a ball out by making a verbal "out" call ("fault" for a serve) followed by the extension of the arm shoulder high in the direction the ball was out. A ball is signalled "safe" or "good" by holding both hands together knee-high in front of the body. A line umpire who is unable to call a shot (usually because a player obstructs their view) signals this by holding their hands beside their head, the upper side pointing in the same direction as the eyes; that call is then the chair umpire's responsibility. Line umpires are also responsible for calling foot faults. When baseline umpires are being used, they watch to see if the server touches any part of the baseline before the ball is struck. Center service line umpires are responsible for calling foot faults when the server steps over the imaginary extension of the centerline. Footfault calls are made with a verbal "Footfault" call followed by a vertical extension of the arm with the palm open. To reduce staff due to COVID-19 pandemic restrictions, the 2020 US Open employed electronic line judges on most matches, excluding those held at Arthur Ashe Stadium and Louis Armstrong Stadium. At the 2021 Australian Open, all matches used electronic line judges for the first time in a Grand Slam event. Off-court officials Referee The referee "is the official who is responsible for assuring that the competition is fair and played under the ITF Rules of Tennis." The referee supervises all of the aspects of play at a tournament including the conduct of players, coaches, spectators, and administrative crew. The referee must be present whenever matches are being played. The referee also "is the final authority on all questions of tennis law..." Any player may appeal the chair umpire's interpretation of tennis law to the tournament referee or deputy referee. In making the decision on an appeal the referee must consider the rulings of fact of the chair umpire, as well as statements of the players and line umpires. The referee's decision is final. The referee is also