Why the D20 System of D&D Isn't as Simple as It Looks
The magic of tabletop RPGs is that they turn storytelling from a one-man show into a team sport. One way most RPGs manage the team is with dice. The dice bring everyone together in suspense, including the host or dungeon master. The dice answer the questions of what players (or non-player characters) can or can’t do, so the whole group can play together instead of arguing.
Of course, there are games like Amber Diceless RPG that avoid dice, or games that use other mechanics like Dread’s off-brand Jenga tower. I’ll set those aside for now, and just talk about dice… although many of the principles in this article can apply.
Today, I’ll do a deep dive into how the famous twenty-sided works in the most famous of all RPGs: Dungeons and Dragons. It’s a simple-seeming mechanic, that gets a lot less simple under the hood.
Going Back to the Granddaddy: Dungeons and Dragons and the 20-sided Die.
The current (5th) edition of Dungeon and Dragon, the world’s most famous tabletop RPG, focuses most mechanics on the roll of a 20-sided die (AKA, the d20). An attack, a save from a spell, or a skill-based action has some set difficulty. You roll your d20, add (or subtract) your modifiers, and if the total is over the difficulty you succeed.
So how does this impact the game? Well, it introduces suspense via chance (unlike the aforementioned Dread’s tower which introduces suspense via skill challenge).
Going deeper, it introduces chance in a particular way. There is a mix of chance (d20) and character skill or equipment (modifiers) and circumstances (difficulty and/or modifiers). This is not that unusual, but it’s the use of a d20 (with critical success or failure rules) that really changes the equation. Here are just a few reasons why how that single die choice and mechanic sculpts the feel of the game.
Extra Randomness
For one, a d20 has no central tendency… i.e., you are just as likely to get a 1 as a 10 or a 20. This contrasts with games like GURPS where you add up three six-sided dice (d6). In the GURPS case, there are more combinations that make 10 or 11, than those that make 3 or 18. That means that rolls in GURPS are less important and skill or situation modifiers influence the outcome more than random chance.
In contrast, D&D rolls are more random. Especially with critical success (20 is a special win) and critical failure (1 is a special failure), that means players are going to have a much wider range of outcomes and are much more likely to experience edge cases. In D&D players novice players are more likely to hit a home run and experienced players are more likely to fumble than in a system that adds multiple dice rather than rolling the one d20.
Now, this additional randomness will affect all players, but it’s going to hit the host/game master (GM) the most because they are responsible for more of the world and more of the characters, and they have to do more planning which is more likely to be thrown off by the randomness.
GMs compensate in a number of ways. Sometimes, they pretend to roll and just don’t, dictating the outcome, or engaging in railroading. Others adopt loose, improvisational storytelling techniques based on character goals and clues, rather than a strict story outline. I personally prefer the second. Other GMs develop a relativistic approach to success and critical success. They do this especially with interpersonal reactions, so a character with high Charisma can’t simply turn a hostile frost giant into a permanent love slave by rolling a 20. Instead, the DMs set a threshold for normal success (frost giant holds off on attacking) and improve it slightly (frost giant refrains from attacking and gives a piece of helpful information).
Simple Probabilities
Another outcome of using a d20 is that you can easily calculate the probabilities. There’s a 5% chance of rolling any one number, and you can easily add up the numbers. If you need to roll a 15 or higher, then you have a 30% chance of success.
Because of this ease of calculation, DMs will sometimes work backward when setting the difficulty of a skill roll. For example, the GM might decide she wants the player to have a 30% chance of success. That would normally be a difficulty of 15. However, she knows the player has a +3 modifier… so to adjust, she makes the difficulty 18.
That sort of calculation is somewhat harder in a game like GURPs is harder to do in your head on the fly. However, with a table of the roll probabilities, you could do much the same thing.
In other games, however, calculating your chance of success is right out. For example, the World of Darkness games (their Mage game is a personal favorite of mine). In these games, you roll a number of 10-sided dice (d10s) based on your attribute and skill ratings. Each die that rolled an 8, 9, or 10 is a success. If you get the number of successes needed, you get success.
Calculating the chance of one success for a single die (30% chance of success), except you can reroll 10s. Throw in multiple dice, and most GMs or players are not calculating their probabilities. You’re not going to calculate on the fly the chance of getting at least 3 successes on 5 dice, including rerolled 10s.
This Word of Darkness system still includes suspense and chance, but it limits your ability to judge exactly how high (or low) your chance of success is. In practicality, this just means that GMs and players both rely more on the game’s written guidelines for setting a game difficulty. The game creators undoubtedly have geeked out, calculated the probabilities, and given some guidance on what’s easy or hard.
Of course, the ease of calculating the success of your roll depends on knowing the difficulty. In many circumstances, the players don’t (unless the DM tells them, which many DMs don’t). So players have a sense of how high they are likely to roll (given a lot of randomnesses), but not how high they need to roll to succeed. That makes the ease of calculating probabilities a moot issue for all but the game master.
Maybe Too simple? Bolting on Additional Rules in D&D
While rolling a single, 20-sided die is simple to understand, it was clearly too simple for all D&D needed to do, so they have bolted on other rules.
Perhaps the most elegant addition to the standard d20 is rolling advantage (roll 2 d20s and take the highest) and disadvantage (roll 2 d20s and take the lowest). These can be used to model favorable circumstances (including brilliant ideas or help from other players) or unfavorable circumstances. It’s a fairly straightforward extension of the single d20 roll, but it is already at the point where people who don’t like to square decimals in their heads aren’t going to calculate the odds of success.
Of course, there are other rules. Some spells or effects allow you to add an additional four-sided die to your d20 roll. With spells, sometimes you roll to attack the other player, and sometimes you force them to roll against you, so you change who’s rolling. Sometimes you can reroll, but you have to take the second roll (you’re not choosing the highest, as in advantage).
Systems like an advantage, adding d4s, rerolling, and changing who rolls have 3 effects.
They create variety, so it doesn’t feel quite so much like you’re doing the same thing over and over again.
They reinforce the idea that your abilities have a unique impact on the world because they force a change in how the game is played. Having to physically roll another die or reroll or force someone else to roll makes those abilities feel more real.
They undermine simplicity. As long as you’re just rolling a single d20, Dungeons and Dragons is a game you could pick up in a few minutes. Keeping track of which abilities do what, however, turns into a game of looking up in appendices what ability does what. All easy calculation of your chance of success is thrown out the window.
That last one is not to be overlooked. The complexity of D&D’s modifiers is not something you get past, because there are simply too many abilities and spells triggering different game mechanics to keep in your head. Even experience (and professional) DMs like Matthew Mercer of Critical Role still have to look up abilities to be sure they know how they should be used.
The Big Trade-off In Roleplaying Games and Dice Mechanics
So in the end, Dungeons and Dragons takes a simple mechanic and makes it complex.
This is ever a trade-off in roleplaying games. You can have a simple game/conflict resolution mechanic. Your game will be quick to learn and your chance of success easy to calculate. On the other hand, the game side of your RPG will be pretty repetitive and the abilities you use will all have similar game effects and kind of blend together.
On the other side of the spectrum, you can have a complex die-rolling process (or a simple one modded until it’s complex). That makes the game harder to learn, and often slower to play. On the other hand, there is more variety in the actions you take. Abilities or conditions that trigger a different mechanic have more weight and feel more real.
And of course, there are games in between. One of the most excellent examples of this is Apocalypse World and the games it inspired. The core mechanic is rolling two six-sided dice and adding them together with any modifiers. This, in and of itself would put the game in the ‘rules light’ or simple category of RPGs. However, if you get an in-between result (7-9), the player succeeds but has to make a difficult choice. This choice depends on the context, upping the complexity of the game and requiring some appendix searching (especially for new players). However, adding a difficult choice to the rolling process allows the game to differentiate abilities and make rolls seem different and fresh).
How I Used All These Ideas in Designing My Own RPGs.
I have been developing an RPG called Tox, from whence sprang the fictional setting of The Stitched Realms.
Initially, I did use a fairly simple method of rolling. I found myself making exceptions and bolting things on. I realized I didn’t want a really simple game (a la Doctor Who or No Thank You Evil) because I wanted players to have to think strategically and make decisions that tailored gameplay to their characters.
On the other hand, I didn’t want a lot of one-off rules or exceptions that needed to be looked up. Once you know the mechanic, I didn’t want you to have to pause the game to look up who to roll this particular type of roll. If done right, a more complex initial mechanic, in a system with fewer but more distinct abilities, could actually play faster than a simple mechanic that had to be modded in innumerable ways by innumerable abilities.
That left me with an unusual game mechanic. At first, I was worried, because it was not as simple as D&D appeared to be. Then I realized, after getting a lot of feedback, that D&D is not as simple as it appears. I accepted that players wouldn’t be able to calculate probabilities on the fly. Many players aren’t inclined to calculate probabilities, and in many cases, it wouldn’t be straightforward in D&D (and other games) for those that were so inclined.
The most important questions for dice in an RPG are 1) Is rolling the dice exciting? and 2) Do the dice systems reflect the game world and player choices in a way that feels real and engaging to the players?
I optimized for being able to answer those two questions, and then cut away any complexities that didn’t help me answer yes to them.