words formed in the play, and judges (human or computer) are used, so players are not entitled to know which word(s) are invalid. Penalties for unsuccessfully challenging an acceptable play vary in club and tournament play and are described in greater detail below. End of game Under North American tournament rules, the game ends when either: One player has played every tile on their rack, and no tiles remain in the bag (regardless of the tiles on the opponent's rack). At least six successive scoreless turns have occurred and either player decides to end the game. Either player uses more than 10 minutes of overtime. (For several years, a game could not end with a cumulative score of 0–0, but that is no longer the case, and such games have since occurred a number of times in tournament play, the winner being the player with the lower total point value on their rack and thus a score less negative than the opponent's.) When the game ends, each player's score is reduced by the sum of their unused letters; in addition, if a player has used all of their letters (known as "going out" or "playing out"), the sum of all other players' unused letters is added to that player's score. In tournament play, a player who goes out adds twice that sum, and their opponent is not penalized. Examples Plays can be made in several ways (in what follows, it is assumed that the word JACK has been played on a previous turn; letters in parentheses represent tiles already on the board): Adding one or more letters to an existing word, e.g. (JACK)S, HI(JACK), HI(JACK)ING, (JACK)FRUIT. "Hooking" a word and playing perpendicular to that word, e.g. playing IONIZES with the S hooked on (JACK) to make (JACK)S. Playing perpendicular to a word, e.g. YEU(K)Y through the K in (JACK). Playing parallel to a word(s) forming several short words, e.g. CON played under (JACK) simultaneously forming (J)O and (A)N. Any combination of these is allowed in a play, as long as all the letters placed on the board in one play lie in one row or column and are connected by a main word, and any run of tiles on two or more consecutive squares along a row or column constitutes a valid word. Words must read either left-to-right or top-to-bottom. Diagonal plays are not allowed. Scoring The score for any