TL;DR:

Every tabletop RPG needs a way to decide if an action succeeds; that’s its core dice mechanic. On the surface, all dice just generate randomness, but the way that randomness is expressed shapes the whole experience. This first post defines what a core mechanic actually is, why simplicity and modifiability matter, and sets up a short series exploring how different dice systems create different vibes and outcomes.

Introduction

I’ve been asked to review a lot of systems lately, and I’ve noticed myself drawn immediately to an analysis of the core mechanic of the game in question. Long may this continue, but wouldn’t it be helpful, thought I, if there was some well thought out way for me to sort and identify techniques, using common language and a clearly developed pitch regarding what does what well?

Well, soon there will be. This is part one in a mini series that’s going to be a little self indulgent exploration of core dice mechanics.

What is a Core Dice Mechanic?

All RPGs, at least all the ones I’ve played or seen, involve a gameplay mechanism for introducing uncertainty over the outcome of player actions, and the uncertainty is the point. Without it, there’s no tension or surprise, and no sense that player choices might actually matter.

So to be clear, this uncertainty doesn’t just randomise outcomes for the sake of it, it injects suspense so nobody (especially not the GM!) can predict what’ll happen next.

Now often there’s one overriding procedure for this upon which all other sub mechanics are related to. A core mechanic therefore, is the single recurring way a game decides success or failure. Nearly every roll eventually points back to it.

All TTRPG’s needs a way to answer the question, “Does the action succeed?” But how that answer is generated, and how it feels is the heart of a well defined system.

Versatility, Not Complexity

Take for example, D&D 5e: Roll a d20, add your relevant modifiers and aim to meet or beat a DC set by the GM. This same mechanic is then recycled in combat in the same way: roll a d20 add a different modifier, and meet or beat the target’s AC.

In this way, you could argue that 5e’s mechanic is both simple and versatile. Which is important, because when you have lots of wildly different methods and processes to follow, then you’ve entered the territory of “complicated” and it’s cousin; “confusing”.

I’m a firm advocate that that’s something to be keenly avoided because any jock with an imagination can keep piling processes and mechanics onto a game system, but a talented designer knows it’s more about killing your darlings, and whittling away the chaff to unearth of the gem at the centre.

Modifiable, Not Messy

Good core mechanics should also be neatly modifiable to reflect tweaks that the GM might like to make to your odds of success. Let’s be absolutely clear here - fundamentally, that’s all you’re trying to accomplish – reducing the odds of success from 66% to 33%, or whatever you need to do.

So again, the core mechanic should lend itself to simple modifications: to this end, in 5e for example, we have advantage/disadvantage (roll two d20 and keep the best/worst result) and adjusting the DC according to what the GM deems fair. Other systems go their own way, but they all should have comparable functionality to this.

There is a trap there connected to modifying core mechanics, that is: don’t overwhelm the core mechanic with variable options. Options create analysis paralysis and stress. If you’re running a game, you’ve got enough to think about without repeatedly having to decide which lever to pull to achieve the same impact of affecting the odds of success for a given roll.

So to put it simply; you can tweak difficulty and/or tweak circumstance but anything more than that, and you’re pulling too many levers for one simple outcome.

Thesis: One RNG Is Much The Same As Another RNG - Right?

This brings me onto what I really want to explore in this mini series. Are all core dice mechanics doing the same thing? Are they interchangeable or do they lend themselves towards different subsystems? Do they vibe differently and affect the game’s tone?

I suspect not. But more than this, I expect it'll be interesting to deep dive into these questions.

In the meantime, I had a run down of my games shelf as well as all the games I’ve reviewed this year and created a list of all the different expressions of key elements of the dice loop for core mechanics:

Dice used
- d20
- d100
- d6
- Step dice
Objective
- Roll over
- Roll under
- Count number of successes
Degrees of success/failure determined by
- Not applicable
- Critical success/fail
  - Highest/lowest natural result
  - Rolling a “double”
  - Rolling over 90th percentile of stat
- Over/Under TN/stat by absolute amount
- Over/Under TN/stat by percentage
- Number of successes rolled
Variables dictated by the GM
- TN
- Positive/negative modifiers to dice result
- Step dice rolled
- Dice pool size
- Advantage/Disadvantage
Variables dictated by character stats
- Positive/negative modifiers to dice result
- TN
- Dice pool size
- Step dice rolled

Phew, there’s a lot there to consider. Definitely one for another day me thinks.

Conclusion

This was just part one, an appetizer so to speak. Next time, I’ll start breaking these down. Modifiers, targets, roll-under, roll-over and see how those tiny shifts in probability shape entirely different kinds of games.

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