In the game of Scrabble, a challenge is the act of one player questioning the validity of one or more words formed by another player in the most recent turn. If one or more of the challenged words is not in the previously agreed-upon dictionary, the challenged player loses his turn. If the challenged words are acceptable, the challenger loses his turn. If the challenged player has no tiles left the game is over.
In tournament play, a player challenges by neutralizing the game clock and announcing, "Challenge." Both players must refer to word judge software, or request an adjudicator if one is unable to do so. Depending on the rules in force, there may be different consequences for a challenge. There are three current variations: single challenge, double challenge and penalty challenge.
In single challenge, if a player places a word and his opponent wishes to challenge, the challenger may do so with no penalty. If the word is valid, it remains on the board. If it is invalid, it is removed from the board, the challenged player gets back exactly the same letters as he had, and the turn is noted as a pass with a score of zero points. The challenger receives no penalty (point deduction or loss of a turn) no matter if the challenged play is valid or invalid.
A suggested flaw of this is that a player can challenge any word at any time, even words he knows to be valid; since tournament games are usually timed, this can be used to unfairly gain additional time to think about his own next turn.
Double challenge is most widely used in North American club and tournament play. Suppose player A makes a play, then player B challenges. If the challenged word(s) are found to be acceptable, player B loses his or her turn. If the challenged word(s) are unacceptable, player A removes his or her played tiles and forfeits his turn. In NASPA tournament play, only exception occurs when the first play of the game misses the center star, in which the opponent may challenge the play off the board, regardless of its validity.
A criticism of double challenge is that it is more conducive to bluffing. A player may play a word he or she knows is invalid in the hope that his or her opponent will not risk a challenge.
In penalty challenge, a fixed amount of points for the penalty is agreed on before the game, either by the two players or more often in tournament play, by the organizers. If a player challenges his opponent's turn and it is invalid, the letters are taken back as usual. However if the word is valid, the word remains on the board and the challenger loses 5, 10 or however many points the agreed penalty was. This can either be given to the opponent or subtracted from the challenger's score, depending on the agreed rules.
This is a compromise between single and double challenge, with many of the strengths and weaknesses of the two. Players are unlikely to challenge a word they know just to gain thinking time, as it would cost them some number of points. On the other hand, the potential loss of points of a failed challenge may discourage a player from challenging a word they believe to be invalid, increasing the chances of an invalid word remaining throughout the game.
Scrabble and Scrabble-like game sites on the Internet such as Yahoo! Games' Literati, Internet Scrabble Club and Lexulous offer automatic verification as an option, which makes it impossible to play an invalid word. The Facebook version of Scrabble and Zynga's mobile game Words With Friends make automatic verification compulsory without imposing a penalty for wrong guesses or a limit on the number of attempts, allowing players to guess repeatedly at words until the computer verifies one to be acceptable.
On the Internet Scrabble Club, where multiple languages are available, automatic validation allows players to play in their second language or even languages they have no experience with, without the fear of an invalid word ever being played.
Lexulous also has a "strict challenge" option in which a player loses a turn as well as 20 points on a wrong challenge.