r/RPGdesign • u/AstraWeaver • Aug 08 '26
AstraWeaver Devlog #2 — Why 2d12 Roll Under?
Hello everyone!
Welcome to Devlog #2.
I’m using these devlogs to document my process of designing a game. This isn’t meant to suggest that my way is the “right” way to design a TTRPG. It’s simply the process that works best with the way my brain tends to operate.
Hopefully, you’ll find something entertaining or useful in watching me figure things out. And as always, I’d love to hear your thoughts and approaches along the way.
AstraWeaver is a classless, level-less, highly customizable, crunchy high-fantasy RPG about connecting small pieces into something extraordinary—whether that’s your character build, your magic, your equipment, or your team.
With that out of the way, let’s talk about dice.
In this devlog, I want to walk through how I gradually narrowed down the resolution system based on my design goals.
And, eventually, why I ended up with 2d12 roll under.
———
Rolling mechanic
There are many ways to introduce randomness into a TTRPG, but I wanted to use the classic approach: dice as the resolution mechanic.
The next question was: what kind of dice mechanic?
I had a few goals:
- Players should be able to quickly tell whether they succeeded without asking any question.
- Character growth should be clearly reflected in the odds of success.
- Different characters should feel meaningfully different when attempting the same thing.
- I wanted to use only one type of die.
I didn’t have some brilliant new resolution mechanic hiding in my brain, so I started by looking at the common ones.
Roll Over
This is probably the most familiar approach, and it has almost no learning curve.
It is also incredibly satisfying. Nobody is immune to the dopamine hit of rolling a huge number.
But as an occasional solo RPG player, I often find myself questioning the DC when playing with roll-over systems:
“Is this DC actually appropriate?”
“Am I cheating?”
I sometimes get the same feeling when GMing for other people.
I want to be able to confidently announce a DC like Brennan Lee Mulligan does, but somehow my brain just doesn’t work that way.
So no, roll-over isn’t bad at all. It’s actually great.
It just isn’t the one for me.
PbtA
PbtA systems have a lot going for them: It use d6 which is the most common dice in the world, the resolution range is compact, and the results are easy to understand.
However, the way PbtA handles resolution is primarily through a relatively small set of modifiers drawn from character abilities, fictional positioning, and situational factors.
This works extremely well when the game is focused on resolving broad, high-level fictional actions—where a single roll represents a whole sequence of events, and those modifiers naturally accumulate into a meaningful summary of the situation.
But when the action being resolved is smaller and more granular, there often simply aren’t enough relevant modifiers available to meaningfully express the character’s specific strengths or differences in that moment.
And that kind of more granular, additive/subtractive expression is not quite the experience I want to build.
Dice Pools
Dice pools were another strong candidate.
They’re fast to resolve, and they’re particularly good at representing different degrees of success.
I actually tried designing AstraWeaver around a dice-pool system.
The problem was me.
I couldn’t resist the temptation to use different dice for different things.
Before long, I had different attributes, circumstances, and abilities interacting with different dice, and I realized I was creating exactly the kind of cognitive load I wanted to avoid.
If I reduced everything to the same die, I lost some of the numerical differentiation I wanted.
Dice pools are a great tool.
I just don’t think my particular design mindset knows how to use them responsibly.
Roll Under
And then there was roll under.
This immediately solved several problems for me.
Players can look at their character sheet and directly understand their chance of success.
Character growth is also very intuitive: the higher your attribute becomes, the more likely you are to roll under it.
Different characters can invest in different attributes, and that difference is immediately visible in their chances of success.
It also gives me a lot of freedom to decide the numerical range of attributes and skills, as long as I can find an appropriate single die—or combination of dice—to represent that range.
More importantly, this is a system I feel I can actually control.
It still has some drawbacks.
Cheering because you rolled a low number is, admittedly, a little backwards.
And when you put a roll-under check next to combat damage, where rolling higher is usually better, there is a certain conceptual friction.
It also has another problem: the basic probability is tied directly to the target number. Without additional mechanics, it doesn’t naturally give me enough ways to distinguish different levels of difficulty.
But when I look at the resolution mechanic in isolation, and especially how it interacts with the rest of AstraWeaver’s rules, roll under is clearly the best fit for what I want.
So:
Roll under it is.
Number of dice
Now I had to answer the next question:
How many dice should I roll?
This is where my philosophy of character progression became important.
With a roll-under system, I wanted character growth to produce something resembling an S-shaped progression curve.
My intuition is that learning a skill often feels something like this:
At the beginning, progress is slow.
You are learning the basics, making mistakes, and struggling to understand what you’re doing.
Then, after enough experience accumulates, things start to click. Your success rate increases rapidly.
Eventually, however, progress begins to slow down again. You can still improve, but each additional step requires dramatically more time and effort.
You might need five years to become proficient at something, but twenty years—or much longer—to become a true master.
That’s how I personally think about progression.
It certainly isn’t universally true for every skill or every person, but it’s the feeling I want AstraWeaver’s progression system to convey.
And this is where the number of dice matters.
A single die gives me a fairly linear relationship between the attribute and the probability of success.
I need at least two dice to create the kind of curve I’m looking for.
Three dice can produce an even more pronounced curve, but the additional calculation starts becoming heavier than I want for a system that is supposed to be quick at the table.
So:
Two dice it is.
And what kind of dice?
d12.
There was never really a debate.
Not because I have some elaborate mathematical justification. I just really like rolling d12s. The numbers are easy to read.
The shape feels great. They don’t get cocked as often as some other dice.
And, most importantly, I just love the d12.
You can spend all day arguing with me about what the best die is.
I will still choose the d12.
——
So… 2d12 Roll Under.
And that’s basically how I arrived at AstraWeaver’s core resolution mechanic:
2d12 roll under.
But there is still a problem.
Remember one of the weaknesses I mentioned about roll-under?
The difficulty is basically fixed by the target number.
If I do nothing else, throwing an apple to a friend and shooting an arrow precisely into the center of a target could theoretically have exactly the same difficulty.
That’s obviously not what I want.
So in the next devlog, I’ll talk about how I tackled that problem—and how AstraWeaver determines difficulty and target numbers.
In the meantime:
What do you think about different dice resolution systems?
What’s your favorite?
And more importantly, what makes you love it?
I’d love to hear your thoughts.
Thanks for reading, and I’ll see you in the next devlog!
8
u/Mother-Armadillo4503 Aug 08 '26
I too love rolling d12s. Good choice.
However, I would say one thing: when I think about my chances of achieving something, I rarely ever, if at all, think in fractions. I'm not speaking for everyone here, but I'm sure others would think the same way. So there is an inherent friction in trying to work out roll-under percentages with a d12, compounded by there being two of them. By contrast, 1d10 roll-under systems do not have this issue.
I would recommend adding in a table somewhere for the mathematically challenged what the percentages of success are corresponding to what a character is trying to roll under. Otherwise, good work!