There are a lot of GMing skills that transfer seamlessly between systems. But as you move out from whatever your first RPG was and begin exploring other games, it’s useful to remember that this is not universally true. Games are mechanically different from each other, and what works in one system — whether you’re talking about prepping scenarios or running at the table — can fail spectacularly in another. Perhaps even worse is when it doesn’t fail spectacularly, but just quietly degrades the experience. It can be surprisingly difficult to notice when a habit which is “obviously” the way something is done is actually wrecking your game.

A subtle example of this is how you call for a skill check.

Some RPGs, of course, don’t even have skill checks. If you’re used to running systems with skill checks, that’s a big ol’ tip-off that you’ll need to run skill checks differently in this game.

But RPGs that have skills can be broadly broken down into two categories: those with discrete skills and those with overlapping skills.

In a system with discrete skills, each task will clearly fall into the purview of a single skill. This means that the GM can call for a specific check: “Give me Knowledge: Science.”

In a system with overlapping skills, on the other hand, there might be multiple skills that could apply to the current task. For example, a PC might be able to use Knowledge: Science, but they might also be able to use Quantum Mechanics, Stellar Lore, or even Organic Chemistry (depending on exactly what strange phenomenon they’re attempting to analyze).

If a GM familiar with discrete skill systems calls for specific skill checks in a system with overlapping skills, it will either create a much muddier interaction or even end up needlessly neutering PC abilities (if, for example, no one realizes they should actually be making a Stellar Lore check because that’s the skill they actually have). In a system with overlapping skills, what the GM should generally be doing is describing what the skill check is for and then letting the players identify what skill they can use for it.

GM: I need a skill check to identify a weird space rock that’s glowing with some kind of bioluminescence.

Player 1: Can I use Stellar Lore?

GM: Sounds good.

Player 2: Bioluminescence? Could I use Organic Chemistry?

GM: That works.

I, personally, have a weak preference for systems with discrete skills. I like the clean clarity of “give me Skill X,” and it also makes it easier to call for skill checks without first telling the player exactly what the check is for.

But there are definitely advantages to overlapped skill systems. Such systems tend to be used in games with a lot more detail in their skill systems (e.g. GURPS, which has skills for Biology, Chemistry, Naturalist, Paleontology, and Physiology), allowing players a lot more control in describing and distinguishing their characters. Such systems also require players to think about HOW their character is going to approach a particular task or problem, and they can also encourage players to think fiction-first (and discourage GMs from preemptively describing a mechanical solution).

You can also, as seen in our example above, easily end up in situations with people rolling different skills for the same task: Five different people all making the same skill check just skews the probability of success. But successes with different skills can be used to provide different insights into the problem.

The drawback, of course, is that negotiating/identifying which skill(s) to use can be awkward of belabored. There’s not a One True Answer™ here, just different techniques that have different strengths and weaknesses. Understanding those strengths and weaknesses, however, will let you steer towards the strengths and minimize the weaknesses of whatever system you’re currently using.

THE SPECTRUM

Of course, in practice, many RPGs won’t fit perfectly into either the “discrete” or “overlapped” categories. Even systems with heavily discrete skills will likely still have some situations that fall into a gray area. Other systems may have mostly discrete skills, but then a few skill areas with a lot of overlap.

D&D 5^th Edition is an example of this: Most of the skills in the game are very discrete, with the exception of Athletics and Acrobatics, where the distinctions of which physical activities fall into which box can get really hazy. (This is particularly true if you use the, frankly superior in every way and it should be the standard rule, method of allowing skills to be used with multiple ability scores.) And D&D 5^th Edition also has tool proficiencies, which largely operate as a secondary skill system with a ton of vague overlap.

It can be useful to identify these specific features of a skill system and resolve skills appropriately: For example, I tend to call for specific skill checks when running D&D 5^th Edition, except when it comes to Athletics and Acrobatics where I’ll generally say something like, “Give me an Athletics or Acrobatics check,” acknowledging the huge overlap between those skills. And I’ll also say things like, “Give me any appropriate tool proficiency for trying to repair the broken device.”

PLAYER-DEFINED SKILLS

Many RPGs, like Numenera or Dresden Files, have begun using player-defined skills: Rather than having a specific skill list that players choose from, players are free to make up any skill that feels appropriate to them. Such systems encourage fiction-first character creation, and can also create wonderfully evocative character sheets: Lore of the Northern Hills, Greatest Avadrakai Flyer in Parloun, Savage Brawler, etc.

Depending on your predilections, you may find it useful to compile a list of the skills your players have created for their PCs, which you can then often use as a discrete skill list.

On the other hand, I often find it convenient to just treat these systems as being massively overlapped, always calling for skill checks by describing the task and letting players propose which cool skill they’ve created could best apply.

SKILL TREE SYSTEMS

Some RPGs feature skill trees, allowing for characters to purchase both broad skills and then hyper-specialize. For example, a skill like Science might have sub-skills below it on the tree like:

• Physics
• Biology
• Chemistry
• Astronomy
• Quantum Mechanics

And so forth. It’s not unusual for a skill tree system to feature multiple levels, so Chemistry, on the list above, might have additional specialties like Organic Chemistry, Biochemistry, Geochemistry, etc.

Skill tree systems, by their very nature, are overlapped skill systems. (Physics, Biology, etc. all overlap with Science.) But when correctly designed and used they can offer a lot of the advantages of an overlapped skill system, while also smoothing some of the potential awkwardness of calling for skill checks.

For example, a GM can call for a top-level skill check — e.g., “Give me a Science check” — and players can then, with focus, ask if their specific specialty applies. (“Can I use Biology?” “Yes.”)

Alternatively, the GM can call for a more specific skill — e.g., “Give me a Quantum Mechanics check” — and the player can immediately know that they can default to their general Science skill.

What defaulting to a less specific skill means will often be defined by the mechanics of the system (e.g., it’s made at a penalty), but that also serves as a convenient transition to…

PENUMBRAL SKILLS

In addition to gray areas where a particular task could be covered equally well by two or more skills, and therefore you could make the check with either, you can also end up with a situation where a secondary skill could apply, but clearly not as well as whatever the primary skill(s) for the check would be.

In other words, a task that exists in the penumbra of a particular skill. For example:

Player 2: Bioluminescence? Could I use Organic Chemistry?

GM: It’s not exactly your area of expertise, but I’ll allow it.

You can, of course, rarely go wrong by defaulting to yes in these situations. But it may also be appropriate to allow the check, but only with:

• A penalty (“make your Organic Chemistry check at -25%”),
• Disadvantage (or similar mechanic if your system offers it), or
• Limited results (a success with Organic Chemistry just won’t reveal everything a Stellar Lore check can about this weird asteroid)

In the case of offering limited results, you don’t necessarily need to tell the player that this is the case. If your system offers something like a critical or exceptional success, it might also be appropriate to use those outcomes to boost the limited result back to a normal result for our hypothetical organic chemist.