Role-playing game system

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A role-playing game system is a set of game mechanics used in a role-playing game (RPG) to determine the outcome of a character's in-game actions. While early role-playing games relied heavily on either group consensus or the judgement of a single player (the "Dungeon Master" or Game Master) or on randomizers such as dice, later generations of narrativist games allow role-playing to influence the creative input and output of the players, so both acting out roles and employing rules take part in shaping the outcome of the game.

A role-playing game system also affects the game environment, which can take any of several forms. Generic role-playing game systems, such as the d20 System and GURPS, are not tied to a specific storytelling genre or campaign setting and can be used as a framework to play many different types of RPG. Others, such as Dungeons & Dragons, are designed to depict a specific genre or style of play, and others, such as the first and second editions of RuneQuest, are not only genre-specific but come bundled with a specific campaign setting to which the game mechanics are inseparably tied.

Many role-playing game systems involve the generation of random numbers by which success or failure of an action is determined. This can be done using dice (probably the most common method) or cards (as in Castle Falkenstein), but other methods may be used depending on the system. The random result is added to an attribute which is then compared to a difficulty rating, although many variations on this game mechanic exist from system to system. Some systems (such as the Storyteller/Storytelling System and the One-Roll Engine) use dice pools instead of individual dice to generate a series of random numbers, some of which may be discarded or used to determine the magnitude of the result. However, some games (such as the Amber Diceless Role-playing Game and Nobilis) use no random factor at all. These instead use direct comparison of character ability scores to difficulty values, often supplemented with points from a finite but renewable pool. These "resource points" represent a character's additional effort or luck, and can be used strategically by the player to influence the success of an action.