If you've played Dungeons and Dragons, then you know that the basic dice mechanic is a d20. You roll a d20, add a modifier, and compare that result to a Difficulty Class, or DC, for a skill, or the Armor Class (AC) of an enemy, to determine whether you succeed or fail.
There are three types of d20 rolls in D&D: attack rolls (attacking an enemy with a weapon, your bare hands, or certain spells), saving throws (to prevent spells and other bad effects from happening to you), and ability checks (everything else where you want to accomplish something and there's a chance of failure). Rolling a natural 1 (a 1 shows on the die) or a natural 20 (a 20 shows on the die) has a special effect only for attack rolls: natural 1 always misses, and natural 20 always hits and does extra damage. There are a few other places it matters (halfling luck allows you to reroll any natural 1), but many D&D tables houserule additional effects of a natural 1 and natural 20.
The d20 gives a nice, flat probability distribution. You're as likely to roll a 1 as a 20 as a 10. For that reason, it's exceptionally swingy. In many situations, this doesn't matter. If a lock has a DC of 13 and a rogue has +5 with his thieving tools, he has a 65% chance of succeeding. If he rolls a 1 or a 20, he usually doesn't succeed or fail more.
Where that swinginess can produce weird results is with opposed rolls. Let's say your skinny rogue is wrestling with an ogre. The rogue has a -1 on his grappling check, while the ogre gets +4. You would think that the rogue wouldn't have a chance. But because of D&D's famous swinginess, he gets a 26% chance of outwrestling the ogre. That's a lot considering there's a +5 difference.
Many games reduce the swinginess by using dice pools. For example, the Powered by the Apocalypse game engine uses a 2d6 dice pool. GURPS and AGE use a 3d6. 3d6 gives a nice bell curve with results from 3 to 18, centered on 10.5. The problem is that with a standard deviation of 2.96, 67% of the rolls will end up between 8 and 13. The chance of rolling an 18 or a 3 is only 0.5%. In short, it's too narrow a distribution. If you wanted to use 3d6, you'd have to adjust all the bonuses and DCs to make it work.
|Probability distribution curves for 1d20, 3d6, and 2d10. Courtesy of AnyDice.|
|Probability of rolling at least # for 1d20, 3d6, and 2d10. Courtesy of AnyDice.|
Considering all this, how do we implement a 2d10 system in D&D? Simple, any time you roll a 1d20, you substitute a 2d10 instead. If you are doing something that gives you a reroll or extra roll (advantage, disadvantage, luck), you roll 2d10 twice, and take the one (higher or lower) that fits the requirement.
But what about natural 1s and natural 20s? You can't roll a natural 1 anymore, and you roll a natural 20 only 1% of the time. It would be disappointing for critical hits to happen so seldom.
For this we introduce the idea of doubles. If you roll the same number on both d10s, and they are five or less, this is a low double. Count that as critical miss for an attack roll. If you roll the same number on both d10s and they are six or more, that is a high double, count that as a critical hit for attack rolls. Importantly, a natural 1 and natural 20 are not an automatic failure or success except for attack rolls in combat.
We can also use low doubles for abilities that let you re-roll when you roll a natural 1 (for example, halfling luck). Instead substitute low double for natural 1 in that rule. But a low double isn't a 1. If you roll two fives, that's a 10, and it's more of a gamble to reroll that than to reroll an actual 1 any place but for attack rolls, where two fives are an automatic failure.
In addition to this simple conversion, there are some optional rules you can add.
Optional rule 1: Doubles and Degrees of Success
Many DMs allow for degrees of success and failure for ability checks. If your result (roll + modifier) is 5 more than the DC, you are more successful than if you just met the DC, and if your result is below 5 less than the DC, you fail more. A table of degrees of success would look like this.
|Roll range||Degrees of Success|
|DC+10 <= result||3 degrees of success|
|DC+5 <= result < DC+10||2 degrees of success|
|DC <= result < DC+5||1 degree of success|
|DC-5 <= result < DC||0 degrees of success|
|DC-10 <= result < DC-5||1 degree of failure|
|result < DC-10||2 degrees of failure|
- For any sort of knowledge or information check (Arcana, Nature, Survival, Perception, History, Religion, Investigation), the higher your degree of success, the more information you obtain. However, more degrees of failure might actually make it harder for others, as you trample over important clues or confuse your party members with your misinformation.
- With three degrees of success, you don't just climb a wall, you clamber up it at full speed and give anyone following advantage on their climbs.
- With two degrees of success, you climb up at half speed as usual, but those who follow you have advantage on their climb checks.
- One degree of success is just success.
- Zero degrees of success is a failure, but it's a near miss. The DM can allow you to try again without penalty, or even allow you to succeed for a price: it takes longer, or you alert a guard, or you gain a level of exhaustion, or you convince the guard to let you through but you need to bribe him.
- With one degree of failure, you not just fail, you lose ground, so if you can try again at all, it's going to be harder. You fell if you were trying to climb a wall, or you insulted the person you were trying to persuade.
- With two degrees of failure, the task just became impossible, and you're going to have to try another way. You jammed the lock and can't pick it now, or the pipe you were trying to climb collapsed.
Optional rule 2: Extra Dice
|Probability of rolling at least a value for advantage and disadvantage variants, ignoring doubles. Courtesy of AnyDice.|
Optional rule 3: Inspiration
- High double: Both d10s have the same value of 6 or higher.
- Low double: Both d10s have the same value of 5 or lower.
- Ability Check: roll 2d10 + modifier (high double adds 5, low double subtracts 5)
- Ability Check Degrees of Success/Failure: For every 5 above or below the DC, you get an additional degree of success or failure, respectively. What this means depends on context.
- Saving Throw: roll 2d10 + modifier (high double always succeeds, low double always fails)
- Attack roll: roll 2d10 + modifier (high double always hits, crits if would hit anyway, low double always misses)
- Advantage: roll 3d10, pick two (it's always advantageous to pick high doubles and avoid low doubles, otherwise pick the highest two d10s)
- Disadvantage: roll 3d10, drop the highest.
- Inspiration: Gained when roll any double final result. Can be spent after roll to turn into advantage (even if initially rolled with disadvantage), roll an extra d10 if necessary
- Luck point: Roll an extra d10, pick two d10s. (If initially rolled with disadvantage, still remove the highest roll before picking two.)