A good stroke must touch the front wall above a high wooden (often cloth-covered) board before touching the floor. The ball may touch the side walls before reaching the front wall. The player returning a good stroke may play the ball on the volley, or after one bounce on the floor. The play is fast, and potentially dangerous. Lets (replayed points) are common, as the striker should not play the ball if doing so risks hitting another player with it. Matches preferably are observed by a "marker", who has the duty to call "Play" after each good stroke to denote that the ball is "up". Games are to 15 points, unless the game is tied at 13–all or 14–all, in which case the game can be "set" to 16 or 18 (in the case of 13–all) or to 15 or 17 (in the case of 14–all) at the option of the player first reaching 13 or 14. Only the server (hand-in) can score—the receiver (hand-out) who wins a rally becomes the server. Return of service can be extremely difficult, and, in North America, only one serve is allowed. Matches are typically best of five games.