In board games, players may agree to add, subtract, or alter the provided rules of the game. A modification that becomes a permanent feature of a group's play would be considered a "house rule".
Of note, Monopoly is frequently played with slightly different rules to those provided by the manufacturers, to the extent that, according to one commentator "virtually no-one plays the game with the rules as written".
A common use of the term is in role-playing games to signify a deviation of game play from the official rules. The use of house rules is encouraged in a number of official game materials, as a way to personalize the game. Many other games do not explicitly encourage house rules, although house rules are commonly used in casual settings. Games that are played in tournaments typically have very explicit official tournament rules so that house rules are unnecessary. The anime-based RPG Mekton refers to house rules as "changing the laws of physics."
House rules can range from the tiniest of changes or additions to substantial deviations that alter the entire game play, depending on the imagination of the players. Most groups have house rules to some extent. In miniature wargaming, house rules may be used to represent equally unofficial miniature conversions or can be used as scenario specific rules.
House rules date back to the earliest days of role-playing: the original edition of Dungeons & Dragons suggested that players should have a copy of the Chainmail historical wargame for measurement and combat rules and, even more confusingly, it presumed ownership of the Avalon Hill game Outdoor Survival (at the time, Avalon Hill was a competitor to D&D's publisher, TSR, Inc.; later, TSR and Avalon Hill would come under the Hasbro/Wizards of the Coast umbrella). Since many players who purchased D&D did not own copies of Chainmail or Outdoor Survival, they simply made up rules to cover the holes in D&D; many of these house rules later became the basis for Advanced Dungeons & Dragons.