A house rule is a change to a game’s published or traditional rules adopted by a particular group of players. House rules are local: they apply at this table, in this group, for these sessions. They don’t change the game in general; they change the game as played here.
House rules emerge for many reasons. A rule may feel unfair, slow, confusing, or poorly suited to the group’s preferred style. A missing rule may need to be invented (what happens when two players tie?). A group may want to adjust difficulty, remove content that makes someone uncomfortable, or adapt a game to available materials. In role-playing games, house rules are pervasive — most groups modify their system to some degree. In card games, regional variants often began as house rules that spread.
Structurally, house rules sit between the game-as-designed and the game-as-played. They reveal what players actually value — when someone changes a rule, they are making a claim about what the game should feel like. Studying house rules across groups can surface design problems that formal playtesting misses. See also variants, rulesets, and house rules in the card games curriculum.