Ever since I read the Fudge book, I've looked at every game I've ever played differently. I want to write my thoughts about the concept I'm calling, "Rule Coherence." It's this notion where we see games focus on a particular rule or mechanic throughout its game.

The d20 System

The first real games I ever played were based on the d20 System. This system covers a lot of games from the 2000s. It's entire purpose was to create rule coherence by taking the older style of D&D which was not coherent, and solving for that (and some other things).

Here's the basic way the rule they built around: Roll 1d20 + Core modifier + Ability Modifier + Conditional Modifier(s). Compare this to the difficulty.

Almost everything in the game system comes back to this one thing. Saving throws, combat, spells, and skills all fall back to this.

Now, this level of coherence created something interesting. For DMs, it created a mental framework that they could use and improvise around. For players it created a baseline set of expectations they could play around too.

OSR, OD&D, and Its Kin

Now, even though the d20 system had high coherence, there was a revival of the older style games. These had meme-worthy low rule coherence. By low rule coherence I mean there are almost no unifying or similar mechanics to lean on. Attacking is different than spell casting which is different than saving throws, and different for skills.

There was no unifying concept between the mechanics other than a loose notion to represent a "Chance of" something.

So from the rule coherence perspective how would a low coherent game gain favorability over a high coherence game?

Speaking only from the perspective of coherence, low coherence works when it stays simple. In other words, it needs to fit in your head.

These old games, even though everything is different, there aren't many of those things and they stay really simple. The disjointed rules are made up for by their small number and overall straightforwardness.

Let me give a quicker example. In these games, to hit something you roll a d20, there might be a single modifier to it, and you compare it to a number. To see if you hear noises, you might roll a d6 or d20 and compare it to a number. This is uncomplicated and easier to remember than a formula.

What High Coherence Gives

Games with high rule coherence create an opportunity to become easy to learn and play. You might be looking back at my coherence example of d20, but I'll say that's why its only an opportunity.

The other thing it provides is a foothold for the GM or Referee to improvise around. The more that the game works in a single way, the easier it is to make novel scenarios fit in a similar way.

Another interesting thing coherence provides is this idea that you can begin to create modularity. A core game with high coherence often creates an easy and intuitive way to add new hacks, variations, and types of play built on and around the highly coherent aspects of the game.

Mausritter's conditions are a really good example of this. Fudge is a really good example of this.

Now, high coherence can create problems. The risk of favoring rule coherence absolutely is that you may have a coherent system that creates divergent expectations of play. A d20 based encumbrance system would add even more coherence, but would prompt a reaction of, "That doesn't make sense."

Coherence Breaks

I brought up encumbrance and inventory. It's a place that in most systems is a coherence break. That means that it is an area of most games where it will diverge from the rest of the game in terms of how it works.

It is also one of the most house-ruled (ignored) parts of games.

I mentioned before that forcing coherence can break the overall feel of the game. Now, this is interesting because there is still, from the lens of coherence, a way to blend two non-coherent systems.

In the d20 world they create modifiers to your d20 rolls. In Mausritter they use conditions to bridge between their inventory and other mechanics. In 24XX your equipment can avoid consequence. There are lots of examples of these transition mechanics to bridge non-coherent systems together.

I've noticed, though, that these transition mechanics are delicate things to get right. Let me share my ideas on this.

Bolted on Vs Integrated To

Okay, so conceptually coherence transitions fall into two categories, those that are bolted on and those that are integrated to. There is probably more and better ways to describe it, but that's what I'm going with.

Mechanics that are integrated to the coherent ones create a sense of order to the importance to the rules and how they work. You could look at it like one mechanic serves another. An example to a transition mechanic that operates this way is the d20 encumbrance. It operates integrates to the d20 as a conditional modifier. I'll give Troika's opposition roll as an example. It tries to get you back to the modified 2d6 roll, and it breaks a bit of coherence doing it.

Now, transition mechanics that are bolted on are more independent. They don't really serve any other mechanic. They coexist alongside other mechanics. The transitional element allows them to work together, but not in a way that gives preference.

Mausritter's inventory system bolts on to everything else with conditions. Troika's advancement bolts on to the game with marking skills instead of independent XP tracking or something else. d20's magic bolts on with the spell save (Didn't see that coming did you?).

Now, this is all fuzzy, but there's a feel to this that is hard to describe. One of these feels like it is easy to forget, and another feels like it is easy to get wrong.

The integrated transitions are easy to forget, but the bolted on ones feel easy to get wrong.

Coherence or Not?

When I think about coherence as I design games I aim for coherence within distinct elements of play. For example, if I think a major aspect of my game is tactical combat, I want coherence for that. If I, on the other hand, feel resource management is vital to the core game, I want coherence for that.

This sounds obvious, but the challenge comes in when you want a universal system. Now we have to grapple with things like Fudge or the transition mechanics.

To me, I think transition mechanics that are integrated are less favorable than ones bolted on. Again, this is somewhat coupled to the notion that coherence within key gameplay elements is what I'm focusing on, so bolted on will make more sense. The risk of course is that they're easy to forget, and creating transitions is delicate.

There's an element here that is tricky which is the more independent aspects there are, we run the risk of making the game unapproachable. It is a difficult balance.

What I have observed is that games I find elegance in, tend to take an approach of focusing on essentials and not comprehensiveness. This keeps things simple enough to allow for low coherence in a game across its modules, and provides more surface area to bolt onto it another concept.

For a fun read on a similar topic check out Against Dominant Mechanics over at Explorer's Design.