Game System is a RolePlayingGame term. It refers to the set of rules that regulate gameplay and simulate actions that can be performed by the game's characters. The purpose of a game system is to avoid the "Cops and Robbers" problem -- when one player can say "Bang, I shot you!" and the other can say "No you didn't, I dodged so you missed!", no meaningful progress can be made. What a Game System does is set down rules for how well the players can shoot and dodge (among other things), so everyone can agree on what actually happened. Given the sheer number of things that a player might attempt in a RolePlayingGame, {{Game System}}s can be correspondingly complex.

Most Game Systems involve using numerical statistics to mathematically chart a character's abilities, though the exact stats and the nature of their use varies widely between systems. In addition, most systems generally involve some randomness, so that there's an element of chance in most actions -- you can't be 100% certain beforehand whether your attempt will succeed or fail. In TabletopGames, this typically takes the form of UsefulNotes/{{dice}}-rolling, while VideoGames use random number generators instead. In addition, there's typically one person who takes the role of "referee", controlling {{NPC}}'s actions and deciding how rules apply in specific situations. The computer AI does this in VideoGame {{RPG}}s, but it falls to a human GameMaster to do this in TabletopGames -- this, unsurprisingly, means that tabletop {{RPG}}s tend to be much more flexible and fluid than computer-based ones; a program can only handle what it's been programmed to, while a person is much more capable of improvisation.

Game Systems can be divided into two general categories: {{Character Class System}}s and {{Point Build System}}s. A CharacterClassSystem has each player pick their character's "class" (which represents a specific skillset, like combat or stealth) and allows them to advance according to that class's abilities. A PointBuildSystem, on the other hand, allows players to spend character points on whatever SkillScoresAndPerks they want; balance is provided by the cost of abilities and how many points players are allowed to spend, rather than balancing specific classes against each other.

There is a ''lot'' of overlap between the two types of systems, especially in the from of sub-systems. Game designers want to prevent characters of a given class from being identical, so in order to encourage CharacterCustomization, they frequently build a PointBuildSystem ''into'' their CharacterClassSystem. This will usually take the form of some "universal" abilities available to all classes and purchased via point buy, having point-buy abilities within a class so that different characters of the same class may have different abilities, or both.

Originally a Game System and its setting were tightly integrated -- if you wanted to play in a certain setting, or genre, you bought a book or boxed set that described both the system and the setting, and/or an 'adventure pack' that detailed a pre-made storyline for the players to follow. However, over the years the concepts of {{House System}}s and {{Universal System}}s have emerged. {{House System}}s, as the name implies, are HouseRules taken to their logical extreme -- home-brewed systems consisting ''entirely'' (or at least largely) of HouseRules. {{Universal System}}s, on the other hand, are deliberately designed ''without'' a specific setting or storyline -- they usually consist of the basic framework (such as dice-rolling and combat mechanics), but also have rules that allow a player to adapt his favorite novel or computer game for use as a setting.

Specific Game Systems sometimes go through dozens of revisions. Each new version can be expected to either clarify existing rules (if ambiguities have arisen in previous versions), add entirely new mechanics to the system (e.g., adding a new section for vehicular combat rules), or streamline existing rules that have been discovered to be unwieldy or poorly-designed.

Due to the nature of its design, a Game System's rules are not all-encompassing -- some ambiguity is usually left as a "fudge factor" so that a DM can create new rules to suit a particular situation. Such ambiguity can also allow the ''player'' some leeway, by creating loopholes in the game's rules that permit him to do what the rule system would normally prevent. Such loopholes, if found to present an unfair advantage to a player, are usually closed in subsequent revisions.


* The TabletopGame/D20System, originally used by ''TabletopGame/DungeonsAndDragons'' since 3rd Edition. Available under a free license, it has a huge number of settings and spinoff systems related to it.
** ''TabletopGame/MutantsAndMasterminds'' is a superhero-themed system ''heavily'' modified from d20, to the point of being completely PointBuy.
** ''TabletopGame/{{Pathfinder}}'' is the SpiritualSuccessor to D&D 3.5, created after the introduction of D&D 4E.
** ''TabletopGame/MonteCooksWorldOfDarkness''
* ''TabletopGame/BEEREngine''
* ''TabletopGame/{{GURPS}}''
* ''[[TabletopGame/{{Champions}} Hero System]]''
* Guardians of Order's Tri-Stat system, used in:
** ''TabletopGame/BigEyesSmallMouth''
** ''TabletopGame/SilverAgeSentinels''.
* ''TabletopGame/{{Traveller}}''
* By Creator/WhiteWolf, we have the Storyteller system, used in:
** ''TabletopGame/{{Exalted}}''
** ''TabletopGame/{{Scion}}''
** ''TabletopGame/TrinityUniverse''
** ''[[TabletopGame/OldWorldOfDarkness The World of Darkness]]''
*** ''TabletopGame/ChroniclesOfDarkness'' uses the Storytell'''ing''' system, a cousin system.
* ''TabletopGame/{{FATAL}}'', an infamously horrible "adult" RPG.
* ''TabletopGame/{{Fudge}}'' (including ''FATE'')
* ''TabletopGame/LegendOfTheFiveRings''
* R. Talsorian's Interlock system, used in:
** ''TabletopGame/{{Mekton}}'', a HumongousMecha game.
** ''TabletopGame/{{Cyberpunk}} 2020'' (though with a little bit more detailing in the combat).
* ''TabletopGame/{{Paranoia}}''
* The ''TabletopGame/{{Palladium}}'' system, used by ''TabletopGame/{{Rifts}}'', ''Palladium Fantasy'', and others.
* ''TabletopGame/PoweredByTheApocalypse'': Originally developed for ''TabletopGame/ApocalypseWorld'', it has since been adapted to a number of other [=RPGs=], e.g. ''Dungeon World'', ''TabletopGame/MonsterOfTheWeek'', ''TabletopGame/{{Monsterhearts}}'', ''Sagas of the Icelanders'', etc. While not a UniversalSystem ''per se'', it is designed to be lightweight and easily hackable, making it uniquely easy to adapt to any setting and narrative genre.
* ''TabletopGame/{{Shadowrun}}''
* TabletopGame/SavageWorlds from Pinnacle Entertainment Group.
* ''UsefulNotes/{{Unisystem}}''