What Is a d20?
The twenty-sided die (d20) is the most iconic die in tabletop role-playing games. Shaped like a regular icosahedron, it produces results from 1 to 20 with equal probability, giving each face a 5% chance of appearing. In Dungeons & Dragons, nearly every meaningful action a character takes comes down to a d20 roll.
Attack rolls, ability checks, and saving throws all use the d20 as their core mechanic. You roll the die, add the relevant modifier, and compare the total to a target number (called a Difficulty Class, or DC). This simple framework drives the entire game, which is why the d20 has become synonymous with D&D itself.
Natural 1s and Natural 20s
In D&D, a "natural 20" (often called a "nat 20") on an attack roll is an automatic critical hit, regardless of the target's armor class. The attack deals double the normal damage dice and is one of the most celebrated moments at any table. The odds of rolling one are exactly 5%, or 1 in 20.
A "natural 1" on an attack roll is an automatic miss, no matter how high the attacker's bonus. Many groups also play with house rules where a nat 1 results in a comedic fumble, though this is not part of the official rules. For ability checks and saving throws, a natural 1 or 20 does not automatically fail or succeed in standard 5th Edition rules, although some tables apply that as a house rule as well.
Why the d20 Works for RPGs
The flat 5% probability per face means that every result is equally likely, which keeps outcomes unpredictable. A character with a +10 modifier can still fail on a low roll, and a novice can occasionally succeed against the odds. This tension between skill and luck is central to what makes D&D exciting.
The d20 also provides enough granularity (20 distinct outcomes) to support a wide range of difficulty levels without the maths becoming complicated. A DC 10 check succeeds about 55% of the time with no modifier, while a DC 20 requires either a high bonus or a lucky roll. This clean scaling is why the d20 system has been adopted by many RPGs beyond D&D, including Pathfinder, Star Wars d20, and the d20 Modern system.