Systems & Characters
What Is the d20 System? (And Dice Pools, and Roll-Under)
The d20 system is tabletop's dominant engine: roll one twenty-sided die, add a modifier, beat a target number. D&D runs on it, Pathfinder runs on a renovated version of it, and its DNA is in half the hobby. But it's one engine among several, and the others (dice pools, roll-under, percentile, 2d6 mixed-success) produce noticeably different table feelings. Learn the five shapes and every new rulebook becomes skimmable, because under the lore, it's always one of these.
d20 + modifier: the blockbuster engine
One die, big swing, flat odds: every number 1 through 20 is equally likely, so the d20 delivers drama by default; anyone can crit, anyone can faceplant. Your character's skill lives in the modifier, the GM sets a difficulty, and the question is always "did I clear the bar?"
Formalized in 2000 (the "d20 System" was literally a license), it's the engine of D&D, Pathfinder, and countless descendants. Its felt personality: swingy, heroic, legible. New players understand it in one roll, and veterans grumble affectionately about the swing forever. There's a reason it's the default door into the hobby.
Dice pools: grab a handful
Pool systems trade the one big die for several small ones. Sometimes you count successes across the pool; sometimes you read only the highest die, the way Blades in the Dark and Candela Obscura do (6 clean, 4-5 costly, 1-3 ouch). More skill means more dice, so competence feels physical; the handful literally grows.
Statistically, pools cluster toward middling results, which supports the mixed-success storytelling those games love. Felt personality: tactile, textured, fond of complications. Also the engine most likely to make you buy more d6s, as if you needed the push.
Roll-under: your skill is the target
Flip the logic: your skill is the number, and you want to roll at or below it on a d20 (Dragonbane) or d100 (Call of Cthulhu, where "Library Use 60%" means exactly what it says). No modifier math, no GM-set target; the sheet already contains your odds, readable at a glance.
Percentile roll-under is arguably the most beginner-transparent math in the hobby, which is why the horror games that want immersion over arithmetic keep choosing it. Felt personality: grounded, personal, quietly tense; you always know precisely how likely you are to fail, and the die confirms it anyway.
2d6 and the mixed-success revolution
Powered by the Apocalypse games (Monster of the Week and family) roll 2d6 plus a small stat: 10+ clean, 6- trouble, and 7-9 (the bell curve's fat middle, most rolls) succeeding at a cost. The engine mathematically guarantees complications, which means the story bends on nearly every roll.
The newest wave hybridizes freely: Daggerheart's 2d12 Hope/Fear reads success and story-tone off one roll, and Edge of the Empire's symbol dice go further still. The industry's direction of travel is clear: every roll should move the story, not just the outcome.
Which engine is "best" is a genre question, not a quality one: d20 for heroics, pools for texture, roll-under for dread, 2d6 for momentum. The good news is that reading this page was the hard part; the rest is an evening at a table per engine, which is a far better syllabus.
Frequently asked questions
What games use the d20 system?
D&D (every modern edition), Pathfinder, and a large family of licensed descendants from the 2000s d20 boom. Broadly, any game whose core roll is d20-plus-modifier against a target number is running the lineage.
Is the d20 system the same as D&D?
D&D is the origin and flagship, but the d20 System was an open license others built on; Pathfinder famously grew from it. All D&D is d20; not all d20 is D&D.
What's the difference between d20 and dice pool systems?
d20 rolls one swingy die and adds skill as a modifier; pools roll multiple dice where skill adds dice to the handful. d20 produces flat odds and big drama; pools cluster toward the middle and generate mixed outcomes constantly.
What does roll-under mean?
Your skill number is the target: roll at or below it to succeed, on a d20 or a d100 depending on the game. No addition, no GM target-setting; the character sheet is the probability table.
Which dice system is best for beginners?
All of them onboard in one session; they're different flavors, not difficulty tiers. 2d6 PbtA is the smallest rulebook, percentile is the most transparent math, and d20 has the most tables to join, which in practice is the deciding factor.