Decades ago, I read game designer Steve Jackson explain why he swapped the d20 to-hit roll in Dungeons & Dragons for the 3d6 roll used in his alternative combat system Melee (1977). Steve considered the 3d6 bell curve so superior that he trashed the d20 without a second thought. His roleplaying games The Fantasy Trip and GURPS use 3d6 core mechanics. Then, I struggled to grasp Steve’s dislike of the d20.
Now, I understand Jackson’s disdain, but I love D&D. Like esteemed game designers such as Jeremy Crawford and Monte Cook, I find reasons to embrace the d20.
In Cook’s designer’s notes for his Numenera RPG, he describes the d20’s flaw. “Using the d20 introduces a great deal of randomness into a game. It’s difficult to use a d20 as a task resolution die and still have character aspects play a big part in success or failure without all of a sudden finding yourself using pretty big numbers.” He gives an example like this: Suppose two archers try to hit a bullseye by rolling a 20 or higher. An untrained person with a decent 12 Dexterity gets +1 and hits 10% of the time. In comparison, a 12th-level ranger trained in the bow and boasting an 18 Dexterity gets a +8, but still only hits the bullseye 45% of the time.
D&D games show this dynamic when the DM asks everyone to roll an Intelligence (Arcana) check to recognize ancient sigils, and then the brainy wizard fails while the barbarian knucklehead succeeds. That outcome may seem funny the first time, but similar scenes play frequently and can feel disappointing. Instead of rewarding the player who chose to make a character good at something, the d20 roll often makes experts look inept.
If D&D used bigger bonuses, then experts would get a boost. Suppose the expert archer gained a +25 and hitting the bullseye required a roll of 30. Now, the sharpshooter feels more like Annie Oakley. But that arrangement makes difficult tasks impossible for unskilled characters when we really want success to become rare.
Instead of using big modifiers, fifth edition’s bounded accuracy uses modest bonuses that give every character a chance of success at the price of making experts inconsistent.
Monte describes an alternative. “Now imagine that you used 2d10 instead. 2d10 gives us a more normal distribution. In other words, you end up with a much better chance of getting a 10 than a 20. Using the same bonuses, the archer still hits the bullseye 45% of the time, but the unskilled guy only 3% of the time. That makes more sense.” With a 2d10-based game, clumsy newcomers at Faerûn athletic competitions luck into fewer medals. (In one of my very first posts, I grappled with a related issue.)
This more natural range of outcomes leads game designer Steffan O’Sullivan to write, “I’m not fond of dice systems with a flat distribution. I’m solidly in the bell-curve camp.” O’Sullivan created the Fudge RPG, which became the basis for the popular Fate system. Both games use a set of four special 6-sided dice marked on two sides with a plus (+), two with a minus (-), and two blanks. “When you need to roll dice in Fate, pick up four Fate dice and roll them. When you read the dice, read every + as +1, every blank as 0, and every – as –1. Add them all together. You’ll get a result from –4 to +4, most often between –2 and +2.” No roll requires counting past 4, so even little kids add the results easily. “The fewer mathematical calculations used to figure out a dice result, the more likely you are to stay in roleplaying,” O’Sullivan writes. “So Fudge Dice were born, and I like them a lot. They’re a joy to use and don’t slow the game down at all, one of my early design goals.” The system’s bell curve makes results of -4 and +4 rare, but possible. So a +4 (1.23%) matches my real-life chance of a bullseye, while an Olympic archer scores a bullseye on any roll better than -4.
Despite the virtues of the bell curve, Monte Cook still opted for a d20 for Numenera and D&D creators Gary Gygax and Dave Arneson chose the d20 for to-hit rolls and saving throws. The d20 rolls beautifully, it generates a big range of numbers without adding, and the icosahedron feels deliciously different from the bland cubes in countless games like Monopoly. Gygax became particularly enchanted with the exotic new dice from Japan.
Most importantly, d20s yield predictable odds compared to mechanics that combine multiple dice. Monte Cook explains, “If you’re using a system where the GM has to assign a target number for a task, it’s a lot easier to do that on the fly with a d20 than, say 3d6 or 2d10. Why? Because with a d20, the difference between, say, 17 and 18 is the same as the difference between 8 and 9. They’re basically just 5% increments. With a bell curve, that’s a lot harder to figure for the GM, particularly on the fly.”
In 1974, D&D lacked ability checks. To decide between success and failure, Gary Gygax suggested that DMs estimate the chance of success, and then improvise a roll that fits the odds. A d20 roll made the math easy.
Playing D&D means learning to embrace the d20’s swings. To help gamers love the d20, D&D’s current rules architect Jeremy Crawford offered advice on the October 3, 2019 episode of the Dragon Talk podcast. “Any time the d20 is in the mix, that is a swingy die so get ready for the unexpected. What I encourage groups, players and DMs alike to do, is rather than viewing that as something to chafe against or be unhappy about, embrace it as a storytelling opportunity. Over the years, the longer I play D&D and DM D&D, the more I have come to love the unpredictability of the d20, because so often it will create moments that will challenge the DM and the players to really stretch their storytelling ability to come up with a fun reason for why this transpired. Why did the ace rogue who triggered this battle, why did she end up going last?
“When the d20 throws you a curve ball, catch it and follow through with the curve. Just see where it leads you rather than saying, ‘this is dumb’ or ‘this isn’t how it should play out.’ No, in D&D, what the d20 does is really showing how this is going to play out. Let’s ride it and see where this craziness goes.”
If I were magically transported back to a version of 1974 that somehow lacked Gary Gygax and Dave Arneson, would my attempt to bring dungeons and dragons to the world use a d20? I might choose Fudge Dice, but I would never stop giving those new icosahedrons from Japan forlorn looks.
Related: D&D and the Role of the Die Roll, a Love Letter
When You Describe Outcomes, Flatter Your Game’s Heroes and Monsters
In D&D, Letting Everyone Roll Certain Checks Guarantees Success, So Why Bother Rolling?
Gamers needed the how-to. The original D&D rules read as a summary for people who already knew how to play. D&D arrived as a companion to a miniature battle game called Chainmail, and the rules built on a foundation of turns and moves. Gary Gygax’s peers felt comfortable with rules for inches of movement and for how many 10-foot squares a character could search in a 10-minute turn. To Gary’s audience, D&D made sense. But the rule books confused folks accustomed to rolling dice to see how many squares a wheelbarrow could move.
Ken St. Andre wrote T&T—Tunnels & Trolls—because he found the D&D rules “nearly incomprehensible.” He describes T&T as having the same relationship to D&D as “Chevrolet does to Ford.” His explanation of how to play T&T worked for D&D too. “The game is played something like Battleship. The individual players cannot see the board. Only the DM knows what is in the dungeon.” In 1975, games needed boards. (See 
Even though the D&D’s turns and hidden boards felt familiar to gamers in 1974, the game’s wide-open possibilities captured the imagination. In D&D, players could attempt anything. Hardly anyone held to the rigid structure or stayed in the dungeon. A city, The City State of the Invincible Overlord, became the first setting for D&D. (See 
