d20

The d20, or twenty-sided die, is a polyhedral die used primarily in tabletop role-playing games. It is a regular icosahedron with 20 triangular faces, each marked with a value from 1 to 20. On most commercially produced d20s, opposite faces sum to 21, a convention intended to balance the die and distribute results evenly.

In play, the d20 is rolled to determine the outcome of actions. Players typically roll a d20 and add ability modifiers, skill bonuses, and other situational modifiers, then compare the total to a target number such as a difficulty class or an opponent’s defense. A natural roll of 20 is often treated as a critical success, while a natural roll of 1 may be treated as a critical failure, depending on the game's rules. The die is valued for its roughly uniform distribution, with each face having a 1 in 20 chance of appearing on a single roll.

The d20 is part of a standard set of polyhedral dice that includes the d4, d6, d8, d10, and d12. Physical d20s are typically made from plastic or resin, though metal and other materials exist; sizes commonly range from about 16 to 20 millimeters across. Numbering schemes can vary, but the 1–20 range is standard in most uses.

Historically, the d20 gained prominence with the release of Dungeons & Dragons Third Edition (2000) and the associated d20 System, which used the open gaming license to encourage wider adoption. Since then, the d20 has become a common reference in many role-playing games and related gaming contexts.