GURPS is a skill-based roleplaying game. If you understand how skill levels work mechanically, you can run or play a game. It’s easy to lose sight of the skill system because GURPS has elaborate options for character customization. But, if you’re feeling overwhelmed, you can always strip the system back to skills in order to create a manageable gaming experience.

## The GURPS Skill Triangle

In play, GURPS skills are driven by three factors: the base skill level, the effective skill level, and the task difficulty modifiers. These three elements make up what I call the GURPS Skill Triangle. If you understand how the Triangle is put together, you can easily improvise challenges in GURPS.

### Base Skill Level

The base skill level is what level the character has listed for a skill on their character sheet. The *Basic Set *gives the following descriptions for what base skill levels represent (paraphrasing from p. B172):

- Ordinary folks have base skill levels ranging from 8 to 13. Skills important to the character’s profession tend to be at level 12 or 13; rarely used skills are often at level 8 or 9.
- Experts have skill levels that go higher. In general, even a master in the field will usually max out a specific skill at 20 or 25, preferring to study complimentary skills instead of pushing an elite skill above level 25.

Base skill levels below 10 are poorly known skills; the character can succeed at the task on occasion or with aid, but frequently struggles with the ability. At skill levels 10 or 11, the character is more often successful but their abilities are still inconsistent. At level 12, skills are solid enough to cover most occupational demands (whether that be swinging a sword, negotiating a settlement, or navigating a bureaucracy). And, at levels 14 and above, the person demonstrates mastery of their ability.

### Task Difficulty Modifiers

Of course, some tasks are more difficult than others. The second leg of the Triangle, task difficulty modifiers, is a catch-all for the situational modifiers that apply to a given success roll. This can include bonuses for using complimentary skills or easier-than-average conditions, penalties for poor equipment or working at a different tech level—anything that changes the odds of success. GURPS has rules that codify the modifiers for a variety of situations, but GMs can also use generic task difficulty modifiers to indicate that a task is more or less challenging.

The default task difficulty, +0, represents using the skill under normal adventuring conditions. Ordinary, everyday situations are less stressful and therefore get bonuses to reflect the less challenging circumstances. By contrast, tasks that would give pause to even brave adventurers receive task difficulty penalties in order to reflect the challenge of the situation.

*Basic Set* pp. B345–B346 describes the range of typical task difficulties, from +10 (automatic success except for flukes of chance) all the way to -10 (impossible tasks that no sane person would attempt).

### Effective Skill Level

The third side of the Triangle, effective skill level, is the result of combining the other two sides. The base skill level + the sum of all the relevant task difficulty modifiers results in the effective skill level.

Players make success rolls against the effective skill level of their character, so effective skill levels correspond directly to the chances of success (the full table is listed on p. B171):

- Below effective skill 8, the odds of success are 16.2% or less (depending on the specific skill level).
- Effective skill 8 will succeed 25.9% of the time.
- Effective skill 9 will succeed 37.5% of the time.
- Effective skill 10 will succeed 50% of the time.
- Effective skill 11 will succeed 62.5% of the time.
- Effective skill 12 will succeed 74.1% of the time.
- The odds of success continue increasing for each additional skill level, albeit at a smaller rate as the levels increase.
- Once a character hits an effective skill of 16, their success is capped at 98.1% (because of the fixed chance of rolling a failure or critical failure).

## Using the Triangle

Because all three sides of the Triangle are connected, GMs can manipulate the Triangle in order to produce the results that a game situation demands.

Let’s say that a PC wants investigate a crime scene. The base skill level is set: just read the character sheet to see what the PC has for Investigation. The GM can set the task difficulty modifier based on how complex the scene is, which will produce the effective skill level for that situation.

But, the GM can also work backwards. Let’s say that, for narrative reasons, the GM wants the PC to have just slightly better-than-even odds of finding a specific clue. Perhaps the GM wants to reward the PCs for finding some leads earlier (which narrow down the search) but still wants to emphasize that the PCs don’t have the whole story. In that case, the GM can use the Triangle to figure out how to create the desired effective skill level of 11. If the PC has a base skill of Investigation-15, the GM needs to describe the situation in order to justify a task difficulty modifier of -4. The GM could therefore describe the apparent disorder of the scene, the lack of immediately obvious signs, or the short amount of time the PCs have until the cops arrive and kick them out of the scene.

The GM can keep working backwards through the Triangle in order to flesh out the relevant NPCs. Suppose the player characters are trying to track a bandit through the forest. The players know the task difficulty modifier for that situation: -4 because of poor weather and unfamiliar terrain. If they fail, but they encounter a ranger who can hunt down the bandit easily, it’s clear that the ranger has a much higher base skill because he can absorb the tracking penalties and still have a high effective skill level. The PCs may want to befriend this ranger in order to take advantage of his superior talents!

On the other hand, the Triangle can be used to show that the NPCs aren’t as skilled as the players might expect. If the PCs observe a mage working a ritual, and the mage surrounds herself with lots of props in order to power the ritual but still barely eeks out a successful casting, that tells the players that the mage had a relatively low effective skill despite a high bonus for the task difficulty, which implies that she has a low base skill for ritual magic. If the players are normally cautious, this kind of information about the mage can help them assess how dangerous she would be when cornered.

## Quick-and-Dirty Skill Levels

Finally, the GM can take advantage of the Triangle to simplify non-player character building. Instead of figuring out what skill levels NPCs should have before the game, the GM can use the Triangle to come up with plausible character stats on the fly.

Because the three sides of the Triangle are linked, the GM can use any two of the three elements to estimate the third side. An NPC should probably succeed on a task? Assume the NPC has an effective skill of 14, determine how difficult the task is, and then derive the base skill level from those two factors. Or, if the NPC has a base skill already established but he or she should likely fail at a task in this situation, work out the task difficulty modifiers so that his or her effective skill is 8 or less.

## Applying the Skill Triangle

GURPS often requires GM judgment. By understanding the Skill Triangle, GMs can check their ballpark estimates to make sure that their judgment calls fit the game. Is a task penalty too harsh? Check what effect it has on the odds of success for the new effective skill. Is a base skill level too low for the campaign? Ask what kind of difficulty penalties the character will have to deal with on a regular basis, and see whether the resulting effective skill level is high enough to challenge the players without overwhelming them. Are the characters being challenged enough? Look at what their effective skill is, and adjust the task difficulty modifiers until the players feel a genuine sense of risk.

The Triangle enables GMs to check their work by comparing the game mechanics to the narrative descriptions those mechanics represent. If one side of the Triangle is out of whack, the GM can refine the mechanics until all the elements make sense together.