<P> The officials must ensure that the game runs smoothly, and this encompasses a variety of different responsibilities, from calling the game to player and spectator management . They also carry a duty of care to the players they officiate and to ensure that the court and all equipment used is in a safe and usable condition . Should there be an issue that inhibits the safe playing of the game, then it is the job of the officials to rectify the problem . Quite often, the job of an official surpasses that of the game at hand, as they must overcome unforeseen situations that may or may not have an influence on the game . </P> <P> There are two standard methods for officiating a basketball game, either "two - person" or "three - person" mechanics depending on how many officials are available to work the game . In "two - person" mechanics, each official works either the lead or the trail position . The lead position is normally along the baseline of the court, with the trail position having its starting point at the free throw line extended on the left side of the court facing the basket . Officials change position often during the game to cover the area in the best possible way . As the game transitions from one end of the court to the other, the lead becomes the trail and vice versa . Between the two positions, each is responsible for a specific part of the court as well as two each of the side, base or back court lines . Officials change position after certain calls (mostly when the lead official calls a defensive foul). This allows officials to alternate between positions to increase the speed of play; one official will approach the bench whilst another, usually the closest to the bench at that time, will move to occupy his position . This also prevents one official from always working one particular team's basket throughout the course of the game . </P> <P> In "three - person" mechanics, the court is further divided among three officials, with the lead (baseline) official determining the position of the other two officials . Normally, the lead official will move to the side of the court in which the ball is located (strong side), particularly if there is a "post-up" player in that position . The official that is on the same sideline as the lead official then takes up a position approximately level with the top of the three - point line and becomes the "trail" official, while the third official will stand across the court near the free throw line in what is called the center position . This creates a triangle coverage of the court . Often, the lead will switch sides of the baseline during a play, requiring the trail to move down to be level with the free - throw line and become the new center, while the center will move up and become the trail . As the ball moves to the other end of the court in transition, the lead will become the trail, the trail will become the lead, and the center official will remain in the center (unless the new "lead" moves across the baseline to cover the strong side, in which case the center official and trail would then switch positions .) </P> <P> Basketball is regarded by many fans as among the most difficult sports to officiate . Usually basketball officials have only a brief moment to determine if an infraction has occurred, due to the speed of play of the game and the officials' proximity to the action . Especially difficult is the "charge / block" call as to which player is responsible for contact . Officials often rely on instant replay to review plays . </P>

When did the nba start using 3 referees