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To kick off the series of articles on RPG rules, let’s first discuss dice. Dice rolls in RPGs serve a few different purposes, and it’s fairly important to understand these before trying to come up with other rules.
But first, some gaming conventions about dice. Apart from the six-sided dice used in pretty much all board games these days, any self-respecting games shop will sell you an assortment of dice with a different number of faces. These usually come in the form of platonic polyhedrons, which gives us at least five different dice types to choose from. There are also ten-sided dice, which aren’t platonic solids, but few people will care about that when playing games.
Most games tend to refer to different sided dice by a lower case d followed by the amount of values you can roll with them. Your four-sided die is a d4, then there’s d6, d8, d10, d12 and d20. What you’ll also see quite often is a reference to d100 — while dice with one hundred faces do physically exist1, you’ll usually use two d10 for d100 rolls. One, picked before you roll, is the tens digit and the other is the ones digit. As their face values tend to go from zero to nine, two zeros will mean a result of one hundred.
It’d be entirely possible to create a dice-less role-playing game. As discussed previously, the preoccupation of many RPGs with fighting is in part to do with their evolving from wargames — any game where you play a role, but there is no matching of strength involved would qualify as a good candidate for making dice-less. In a way, the game of Werewolf/Mafia could be interpreted as a dice-less RPG.
If your game does involve a matching of strength, though, then you essentially have two choices. The game will either have a fairly simple mechanic, where one thing always trumps another, and it’s up to you to decide which of these things to pick. Rock-paper-scissors is the perfect example for one of these games, as it’s simplified to the extreme: any of three options will win against one of the others, but lose against the last.
Or you include an element of chance, which is most easily represented by the throw of dice2. The purest example of a throw of dice as the element of chance I’ve seen is in some naval wargames, where the dice don’t influence the underlying rock-paper-scissors mechanics directly. Rather, the dice merely influence the weather — and some ships do better in some kind of weather than others.
RPGs tend to use dice in a more direct manner, though, and that is to pit chance against a character’s abilities.
Procedures for this vary, but you usually add or subtract modifiers from a dice roll, and compare the result with some threshold values. In Rolemaster, for example, you roll a d100, apply appropriate modifiers, and (usually) succeed if the result is equal to or exceeds one hundred. Adding modifiers to dice rolls is such a common thing that games tend to express this as, for example, d20+5 — that is, roll a d20 and add five to the result.