The d20 Is Special

There are a million RPGs out there, and they use a ton of different dice models for their main mechanics. D&D uses the d20, many use only d6s (Blades in the Dark, PbtA), some use only d10s (Warhammer, White Wolf)… but only one passes what I call “the T-shirt test”… that is, only one mechanic is popular enough to be put on a t-shirt, and that’s the d20. Why is that?

As RPG designers, we like to look at probabilities. The d20 is very straight forward and honestly kind of boring - 20 sides, 5% chance for any side to come up, +1 to the roll is +5% chance to succeed. It’s nice and regular, but it’s not inspiring. And yet, it passes the T-shirt test, why? Well, I think there’s a few reasons.

First, yes, it is the main mechanic of the 800 pound gorilla on RPGs, D&D. But I don’t think that’s the only reason it is popular. There are many rabid fans of other systems that don’t go get their system’s main dice on a t-shirt.

The key to thinking about what the t-shirt test means is thinking beyond the numbers. This is hard for us as designers, because we want to break everything down into probabilities to know how likely an action is to succeed at any point. But that’s not what players are thinking when they grab their dice. They’re thinking “Can I hit this dragon?” “Can I dodge this punch?” “Can I convince the starship captain to help us?” The d20’s easy probabilities make it easy for a player to judge the likelihood of success, but nobody is doing the math when death is on the line.

And I think this is the one key reason why the d20 is seen as almost holy - special enough to be worn prominently on a t-shirt. In most games that use a d20 to determine results, it’s not used for anything else. When you pick up a d20, you know something important is going down. The game is at a tipping point. This one die you hold in your hand holds the key to glorious victory or soul-crushing defeat.

And that’s the second key reason why it’s holy. There is only one. You don’t roll 10, you roll one. In software development we call this difference pets vs cattle. You care more about your one pet than you do about a dozen cattle. When you roll 10d6, they’re cattle - you don’t care about any one die very much. When you roll a single d20, it’s a pet.

Many times in your own games and games streamed online, you’ll see players swap out their d20, or put their d20 in “jail” for rolling poorly. This is a pet. It’s a singular die almost given a life of its own, in need of special attention and rituals. This is why it is on t-shirts.

You don’t have to choose d20 for your game, and I totally understand why most indie game designers don’t. It’s hard to be seen as going along with the 800 pound gorilla. It’s hard to choose a flat distribution. But think about the t-shirt factor of your choice. Maybe it’s not a pet, but consider avoiding making it cattle.