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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 on the most recent turn. In double challenge (most common in North American tournaments), if one or more of the challenged words is not in the agreed-upon dictionary or word source, the challenged player loses his turn. If all challenged words are acceptable, the challenger loses his turn.
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 play, there may be different consequences for a challenge. There are three common variations: double challenge, single challenge, and penalty challenge.
Double challenge is most widely used in North American club and tournament play. Suppose Alice makes a play, then Bob challenges. If the challenged word(s) are found to be acceptable, Bob loses his turn. If the challenged word(s) are unacceptable, Alice removes her played tiles and forfeits her 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 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.
In penalty challenge (also known as modified single challenge, 5-point challenge, or 10-point 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 director(s). If a player challenges his opponent's play and it is invalid, the letters are taken back as usual and the opponent loses his turn. 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 may cost points or spread. 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.
Holding a play
In club and tournament play, a player may choose to "hold" the previous play, provided that the opponent has not seen a single drawn tile yet, in order to consider whether to challenge. To do this, the player simply announces, "Hold," which prevents the opponent from drawing replenishment tiles for 15 seconds (but may draw thereafter, keeping those tiles separate). The player may decide to rescind the hold, or challenge the play.
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.