Probably one of the most important aspects of game design in the roleplaying genre is that no one die mechanic can handle everything as is. This is not to say a system cannot use a single die, but that the core mechanic cannot handle everything. There are a finite number of formulae one can employ in a game system before it becomes unwieldy. Likewise, too few and it can degenerate into predictability. There is a way to avoid both of these extremes: subsystems. They can be resolved by using the same die roll, but some twist on how those dice are read applies. In effect, a special case in the game.
Perhaps one of the easiest subsystems to spot is the damage roll. Even if a system uses a single die, damage does not require a threshold to be reached to determine success or failure. The roll denotes the severity of the attack. This is a derivation from the core mechanic
The singular purpose of a core mechanic is to impartially determine success or failure of any action. That is what an attack roll represents. The same can be said to apply to other tasks as evident in games such as the d20/OGL system, GURPS, West End Games’ D6 system, and the HERO system. All of these systems have core mechanics to resolve outcomes, but they rely on subsystems to complete actions initiated through the core mechanics. Why is this so?
Perhaps it is easier to conceive of game mechanics as tools. The core mechanic is a general, all-purpose tool. It is meant to do the heavy lifting of the games mathematical model. Subsystems fine tune or address special case situations which stretch or threaten to break the simulation. The importance of this cannot be stressed enough. Too much reliance on a single mechanic requires a lot of explanation for each special case or risk a reliance on player arbitration to prevent the system from exhibiting holes or faults. All systems have their limitations, of course, but their limits should be the edge of the “physics” of the environment represented, making it easier for players to judge when something attempted should be considered verboten.
If, as a designer, your aim is to construct a storytelling platform, the reliance on just a core mechanic is an appealing choice. It is also riskier. A handful of probability mechanics may require a steeper learning curve for the rules, but it is the trade-off for a well-defined game. This last point bears repeating in other words: this genre is still a game. While some of the play is in the language and the imagining of the tale, the rules should also accommodate fairness to give players an equal voice/weight in the story’s unfolding narrative. Games that lack enough tools to ensure impartiality are more likely to fall victim to power struggles amongst less experienced players than their subsystem-laden counterparts because of the more free-formed nature of such simulations.
The irony here is that new players need more rules to get a feel for how RPGs are supposed to support a narrative’s reality, but the subsystems that help players do this and grow accustomed to the relative scales of power between one set of abilities and another can be daunting. Give players a more simplified system and they are likely to be overwhelmed by the amount of work required to keep the game (and story) constant, sort of like the terror of the blank sheet some people face when they try to begin a piece of writing. This is why subsystems are vital. By keeping the number of mechanics to a minimum, games can be balanced so that the interactive element of the game is not needlessly overruled by the math and vice versa.
Subsystems might compartmentalize the system, but they make it easier to learn the concepts that make sandbox games so much fun and versatile. One can view subsystems as training wheels for the neophyte, but their true purpose is to alleviate strain on the core mechanic and fine tune the outcomes possible in the wake of the use of the core mechanic — for better or for ill.
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Anatomy of Game Design: Combat Basics
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Anatomy of Game Design: Experience Points