There are two of them, they wear grey shirts and black pants and sometimes they blow their whistles, run over to the table officials (scorer, clock-keeper, press etc.) and do that thing with their hands. These signals are used to indicate to the players and to the table officials when there has been a foul or violation. Signals are also used to start the game, stop the clock, allow substitution etc. There are three categories of hand signals, covering: fouls (illegal contact or interference with an opposing player), violations (illegal ball-handling, exceeding time limits and so on) and game control (tip-off, time-out, reset clock etc.). The following is not a complete list but illustrates most of the common stuff.