Rules as Interfaces


(approx. 800 words)

This post originally appeared on the Pen and Paper Games Forum. Only some markup (markdown) has changed.

Reading Numernera and a few other rule sets reminded me of a (not terribly original) idea that popped in my head ages ago: rules are the interface between players and GM.

By “interface” I’m thinking of programming interfaces in object-oriented design, but the analogy works just as well with Advanced Programming Interfaces (APIs) in applications like Excel, Graphical User Interfaces (GUIs) in any window-keyboard-mouse desktop application, a Web site’s interface of forms and links, or the protocols between remote processes (like HTTP between a Web browser and a Web server). In every case the interface defines a set of concepts or abstractions, operations one can perform on those abstractions, and discrete elements that make up an abstraction or operation. In object-oriented programming one strives for the smallest or narrowest interface that expresses all relevant operations and information; graphic design follows a similar principle which strives for the greatest amount of information relevant for a task without cluttering the display with useless information.

In the case of an RPG, the most common abstraction is a “character”, with attributes/characteristics/statistics, classes, levels, skills, powers, and so forth. Characters typically also have equipment with its own statistics and maybe NPC retainers with some subset of a player character’s attributes. The character typically acts upon an imaginary world when a player describes the character’s actions to the GM. The GM states whether the actions succeed or fail. In cases where consequences aren’t clear-cut the player performs “tests” (a.k.a. “challenges” or “saving throws”). “Combat time” is a special mode wherein players choose among a well-defined list of actions, each with explicit or implicit parameters: Attack with which weapon (and/or associated numerical values)? Move to where and through what? Cast which spell?

So what’s the point? One can at least qualitatively evaluate an RPG by the amount of information required for common actions, or to play the game as a whole. For example, you can tell a lot about a game by the official character sheet. Character sheets for OD&D and BD&D can fit comfortably on an index card; with a single US Letter (8'' x 11'') page one can write numbers bigger and leave empty space for notes. In contrast a D&D 3.5 character sheet can spread over two, three, or four pages depending on equipment carried, number of spells or class abilities, and the degree to which one wants to pre-calculate Attack Bonuses and Armor Classes.

Numenera has an extremely narrow interface, even narrower than the character sheet would indicate.

  1. The Player(s) declare what they want to do.
  2. The GM states an initial Difficulty Level; a 0 means automatic success.
  3. The Player(s) list skills, assets, Effort, abilities, and numenera that may lower the effective difficulty.
  4. The GM agrees (or not) to each item, and states the adjusted Difficulty Level.
  5. If the Difficulty Level is 0, the action automatically succeeds; otherwise, the Player rolls a d20 and succeeds if his result meets or exceeds 3 x Difficulty Level. Certain numbers on the die have special effects.
  6. Players and GM perform bookkeeping related to resources expended and damage taken (which is a little more complicated).

Note that most of the rules complexity comes in at steps #3 and #6. Combat extends the procedure for resolving actions – as in most modern RPGs – with a formal list of actions, combat rounds, and so forth. The other steps are a template for a large number of RPGs, simplified for faster play; in particular, the Difficulty Level usually stays the same from round to round and between attacks and defense. (If you remember my last post, players roll all dice, and an NPC/monster attack means the player rolls to defend.)

Other games simplify by using only one type of die roll (e.g. D6, FATE, HeroQuest, Mutants & Masterminds, PDQ), reducing attributes/skills/assets/etc. into one or two kinds of thing (e.g. FATE Skills and Aspect’s, HeroQuest abilities, PDQ Qualities), or defining conflicts in purely dramatic terms (e.g. Dread, Primetime Adventures, Sorcerer).

Maybe other people might prefer the randomness and patchwork rules of Dungeon Crawl Classics, or the plentiful arithmetic in GURPS. With the number of things already crammed my head I only have enough room for a few simple resolution procedures.