The Core Mechanic
Everything in Keystone Percentile resolves the same way: roll 1d100 equal to or under your skill or stat to succeed. Higher numbers mean more room to succeed — a character with Persuasion 80 fails less often than one with Persuasion 55.
Critical Success
Rolling ≤ 10 is a Critical. Your GM describes an exceptional result — the lock picks itself, the shot finds the one weak point, the argument lands like a hammer.
Critical Failure
Rolling 90 or above automatically fails, no matter how high your skill. A sniper with Ranged (Heavy) 95 still misses on a 94. Your GM adds a consequence: the weapon jams, you trip, you say the wrong thing at the worst moment.
Critical failures apply to attacks, skills, dodge, untrained attempts, and opposed contests. They do not apply to routine, non-stressful checks — recalling common knowledge, crafting without time pressure, or other moments where failure carries no real cost.
The Difficulty Scale
When a task is harder than average, your GM applies a penalty to your target number:
| Level | Modifier | Example |
|---|---|---|
| Standard | ±0 | Normal lock, calm NPC |
| Hard | –10 | Complex lock, suspicious NPC |
| Demanding | –20 | High-security vault, hostile NPC |
| Severe | –30 | Legendary obstacle, extreme conditions |
| Near-Impossible | –40+ | Reserve for rare narrative moments |
Untrained Rolls
You can attempt anything, even without a relevant skill. Roll ≤ (relevant stat – 20), minimum target of 1.
No critical success is possible when untrained. And you cannot improve an untrained skill through use — you must learn it formally.
Opposed Rolls
When you directly oppose another character — Persuasion vs their WIL, Stealth vs their Perception, a grapple, a chase — both sides roll 1d100 under their relevant skill or stat.
Margin of success = your target number minus your roll. Higher margin wins.
Ties go to the initiator — whoever started the action.
If you win and rolled ≤ 10, that is a Critical Victory. Your GM decides the exceptional result.
Example: Your Persuasion is 75. You roll 22 (margin 53). Your opponent's WIL is 70, they roll 40 (margin 30). You win — and it was decisive.
Dodge
Dodge is always available to everyone — raw DEX, no penalty. Roll ≤ DEX to avoid being hit. Rolling 90+ on Dodge auto-fails, just like any other roll.