A dice pool is when you are rolling a different number of dice for each situation. Usually the # of dice depends on one or more properties of the actor. The classic example is adding Dexterity of 3 to Pistols Skill of 2 to roll 5 dice.
Determining success or failure is usually done in one of three ways:
- Additive: Sum the values of all the dice rolled. Used in Shadowrun, West End Games's Star Wars D6. White Wolf's Trinity and Exalted also use this system, where any die showing less than 7 counts as "0", 7-9 counts as "1" and 10 counts as "2." Some games come with special dice that have a 'success' colour on some sides with the others blank.
- Target Number: The GM sets a number for the difficulty of the circumstance; any dice rolled below that number count as "0", any dice above that count as "1". This method is used for White Wolf Games's older Storyteller system. It's often criticized because few people (game designers notwithstanding) understand when it's appropriate to modify the number of dice or to modify the difficulty number.
- Highest Die: Only use the highest value showing on the dice rolled. In contests, compare each actor's highest die, and in case of ties compare their 2nd highest dice, then third, and so on, with absent dice as automatic losses. This system is used in Dream Pod 9's Heavy Gear, Task Force Games's Prime Directive, and Donjon. When two opponents are using dice of the same size, with pools of the same size, the odds are always 50/50.
- Roll And Keep: The player rolls his dice pool, then keeps a certain number of dice; traditionally this is accomplished by making Attribute + Skill the pool, and keeping a number of dice equal to the Attribute score rolled. The numbers showing on the "kept" dice are then added up, and compared either to a static Target Number (no relation) or, in the case of a contested roll, the sum of the other party's kept dice. This system is used for both Legend of the Five Rings and 7th Sea.
One Roll Engine uses something really weird, and has it's own page.
CthulhuTech uses something weirder still. Fantasy Flight Games' RPGs (including the Star Wars Roleplaying Game and Genesys) use something super fucking weird based on overpriced custom dice, but it largely boils down to Additive with multiple running totals per roll. Not to be confused with the quantity of dice Ork players need.
Common variants are:
- exploding dice that show the highest value.
- dice showing their lowest face are counted as a negative value.
- dice showing their lowest face, in the absence of any successes, shows a critical failure.
- changing the size of the dice for actors that are superior/inferior to each other.
- being able to set one or more dice to a desired facing before or after rolling, usually to represent automatic successes granted by supernatural ability.
A traditional pitfall of the system is extremely high variance in potential outcomes leading to a very-random play feel compared to other rolling systems. This problem is often exacerbated by the popularity of exploding dice within dice pools.
- Shadowrun is considered by some to be the granddaddy of dice-pool RPGs, and uses d6. Older editions used Target Number; 4th ed uses a simple Additive pool (1-4 = 0; 5-6 = 1).
- Inquisitor If a player was trying to perform a dangerous action as part of his turn (all actions had to be declared before the player checked to see how many were successful) he was obliged to then roll less 1's than 6's otherwise he would fumble the action, often with humorous results (guns exploding, premature detonation, setting off alarms or falling to their death were common fates for characters who failed.)
- World of Darkness uses d10 and variable target numbers and exploding 10s.
- Exalted uses Additive d10s with '7' as the target number (1-6 = 0; 7-9 = 1; 10 = 2).
- Donjon uses Highest Die d20s.
- Don't Rest Your Head uses additive to determine success and at the same time highest die to determine the quality of the success (failure always has the same quality -- PAIN).
- FUDGE/FATE uses additive with their own weird dice. A typical roll of "4dF" is exactly the same as "4d3 - 8".
- Diana: Warrior princess uses additive with a target number that depends on the character's level (über characters succeed on 3+). All successes always explode, you keep re-rolling successful dice until there is none.
- Legend of the Five Rings RPG game uses a unique variation of dice pool called "Roll & Keep". The system is fairly simple: you get a dice pool equal to one of your basic stats plus your skill level for a task, but you only get to add up a number of dice equal to your stat, and you try to hit a Target Number. For example, if you want to attack with a katana, you roll a number of dice equal to your Agility and Kenjutsu skill, but you can only count up a number of dice equal to Agility. If you roll a natural 10 on any of your kept dice, you can roll that dice again and add 10 to the total for that dice, and you actually get to keep doing it as long as you keep rolling a 10 on that dice. These "exploding" dice result in some truly incredible moments where a character who normally sucks suddenly gets the luck or divine inspiration to achieve the impossible. (In fact, for every three 10s you get on a given roll, you regain a point of Void, which is the "luck" stat you spend for more dice to roll anyway.) To keep the number of dice rolled to a minimum, 3rd edition capped dice pools to ten, and said every two extra dice you would have rolled become kept dice instead.
It's all bell-curves, just like classic 3d6. Here's the numbers for counting successes on d10s needing 7s (Exalted), and d6s needing 5s (Shadowrun 4e). Both systems use crit failures: in Exalted, if you roll more 1s than successes, and with Shadowrun it's when half or more of the dice are showing 1s and you have no successes. What's a bit messed up with the Exalted-style crit failures is how the more dice you have, because you're more of an expert, the better chance that what few failures you have will be a colossal fuck-ups even with simple tasks. Shadowrun 4e is also weird because you have a greater chance of glitching if you are using an even number of dice.