potted. When the final ball is potted, the player with the most points wins the frame. If there are not enough points remaining on the table for a player to win the frame, that player may offer to concede the frame while at the table (but not while their opponent is still at the table); a frame concession is a common occurrence in professional snooker. Players will often play on even when there are not enough points available for them to win, hoping to force their opponent into playing foul shots by laying . These are shots that are designed to make playing a legal shot harder, such as leaving another ball between the cue ball and the object ball. If the scores are equal when all of the object balls have been potted, the black is used as a tiebreaker. In this situation, called a "", the black ball is returned to its designated spot and the cue ball is played , meaning that it may be placed anywhere on or within the lines of the "D" to start the tiebreak. The referee then tosses a coin and the winner of the toss decides who takes the first strike. The game continues until one of the players either pots the black ball to win the frame, or commits a foul (losing the frame). Professional and competitive amateur matches are officiated by a referee, who is charged with ensuring the proper conduct of players and making decisions "in the interests of fair play". The responsibilities of the referee include announcing the points scored during a break, determining when a foul has been committed and awarding penalty points and free balls accordingly, replacing colours onto their designated spots after they are potted, restoring the balls to their previous positions after the "miss" rule has been invoked (see Scoring), and cleaning the cue ball or any object ball upon request by the striker. Another duty of the referee is to recognise and declare a stalemate when neither player is able to make any progress in the frame. If both players agree, the balls are returned to their starting positions and the frame is restarted (known as a ""), with the same player taking the break-off shot as before. Professional players usually play the game in a sporting manner, declaring fouls they have committed which the referee has not noticed, acknowledging good shots