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In most RPG systems, most non-trivial actions require dice rolls. Most RPGs roll a fixed number of dice, add a number to the die roll based on the character's attributes and skills, and compare the resulting number with a difficulty rating. However, in some systems the character's attributes and skills determine the number of dice to be rolled.
Dice pool systems generally use a single type of die, the most common being six- or ten-sided dice (d6s or d10s), though in some games a character's Attributes or Skills may determine the size of the dice in the pool, as well as their number (Deadlands is an example). While such games may require different sized dice for different rolls, the dice in a given pool are usually all of the same size.
The results on each die may be added together and compared to a target number (as in Over the Edge), or the player may count the number of dice which roll higher than a specified target number, and compare that to a required number of "successes" (as in early editions of Shadowrun or the Storyteller System). In systems using the latter method, the target number required for a success may be fixed (the same for every roll) or variable (assigned depending on the difficulty of a task); the number of successes required may indicate the degree of success, or a minimum number of successes may be required as another means of determining difficulty. Another variation is that a number of dice are rolled, but only some are added together (as in the "Roll and Keep" system used by Legend of the Five Rings and 7th Sea).
Modifying the dice pool
In dice pool systems it is common to add or subtract dice from the pool to represent different circumstances.
Penalties may temporarily reduce the dice pool for one or more skills (for example, a leg wound may reduce the dice pool for actions such as running, climbing, and jumping), and are usually fixed numbers (i.e. the leg wound may reduce a pool by two dice).
Bonuses may temporarily increase dice pools, and usually represent beneficial circumstances (e.g. a character may have a powerful computer to aid her in a database search) or some special effort on the character's part (an effort of will, a strong desire to succeed, or even a supernatural power). Circumstantial bonuses are also usually fixed numbers - the computer above might grant two additional dice, for example - while character traits which grant bonuses are usually an expendable resource, representing special effort. This may take the form of "points" (e.g. "Willpower points" in the Storytelling System), or an extra pool of dice which may be allocated to other pools to augment rolls (e.g. the Combat and Karma pools of earlier edition Shadowrun).
Other complications may be used to simulate luck, superhuman ability or other conditions; a common one is to allow high (or low) rolling dice to be rolled again, the second roll counting as if it were an additional die.
Advantages and Problems
Dice pools are more complex to explain and less intuitive than traditional 'roll over' or 'roll under' systems. On the other hand they are faster to resolve if there are a lot of modifiers as it's very easy to see whether each individual dice succeeded or failed and just count those than it is to add four or five separate modifiers to a dice roll. So whether dice pools are faster and simpler depends on how much detail the system goes into. They also provide players with a greater awareness of the modifiers as if you modify the dice by adding to the pool (or taking away from it) each dice is a concrete thing rather than an abstract number, making it easier for kinaesthetic learners to track. And some people find rolling a large handful of dice fun in its own right.
The other thing dice pools allow is opening up a lot of design space for game designers; if something other than the number on the dice is important you can use the number itself to add richness to the system.
- Exploding Dice as used in the Storyteller System or Diana: Warrior Princess involve counting each maximum result on a dice as a success - and rolling it again to see what the extra dice does. This can create some impressive outcomes where luck favours the character.
- Counting Sets as used in One-Roll Engine games like Monsters and Other Childish Things gives a method of separating whether you succeeded from how successful you are, and introduces a risk element.
- Drama Dice as used in Seventh Sea and Fortune Dice as used in Star Wars: Edge of the Empire allow for abstract one shot rewards where the actual effect is unknown.
- Keep the two highest as used in Marvel Heroic Roleplaying and other Cortex Plus games is a hybrid with roll-over, which uses the dice pool's ability to add modifiers without adding much maths while having the simplicity of traditional roll-over.
- Some results are botches again as used in Marvel Heroic Roleplaying and other Cortex Plus games as well as Star Wars: Edge of the Empire allow a combination of a very successful attempt at what you were trying to and a lot of collateral damage. This also can serve to limit the desired size of the dice pool as more dice produce diminishing returns in terms of success while providing more probability of botches.
- Early versions of the Storyteller System used a version of this by which if you rolled a 1 you suffered a critical failure rather than just had negative consequences. This made rolling botches (critical failures) more likely the higher your skill or attribute as these added more dice. This problem was eliminated in the Revised version of the system and later derivatives by stating that a botch only occurred if no normal successes were scored, and one of the dice came up "1".
West End Games's Ghostbusters role-playing game by Greg Stafford, Lynn Willis, and Sandy Petersen featured a d6 system with an additive dice pool that was applied to characteristics and skills.:189 The first widely successful game to feature dice pools was Greg Costikyan's Star Wars role-playing game (1987), developing the system pioneered the year before in Ghostbusters. (Both games were published by West End Games; Costikyan consulted on Ghostbusters.)
Shadowrun (1989), designed by Bob Charrette, Paul Hume, and Tom Dowd, used a comparative dice pool, where a set of six-sided dice were rolled and each individual die was compared to a target number, with the number of successes then showing the final outcome of the task.:123 Shadowrun was probably the first game to use the "success" mechanic rather than adding the dice together. Dowd refined the dice pool system for White Wolf Publishing's Vampire: The Masquerade (1991).:123 Vampire and Over the Edge (1992) were written by Ars Magica designers Mark Rein-Hagen and Jonathan Tweet respectively, the pair having been impressed by the potential of the dice pool mechanic and each having decided to make their own game based on dice pools. The majority of White Wolf Publishing's subsequent games use variations on Vampire's Storyteller System, and so also make use of the dice pool mechanic.