What is a d20?
A d20 is a twenty-sided die normally numbered from 1 through 20. It is one of the most recognizable polyhedral dice in modern tabletop role-playing games and is closely associated with game systems that resolve checks using a single twenty-sided roll.
Shape and geometry
The familiar d20 is usually a regular icosahedron, one of the five Platonic solids. It has twenty triangular faces, twelve vertices, and thirty edges. Its many faces create a wider single-die numerical range than the d4, d6, d8, d10, or d12.
How a d20 is read
After a roll, the numeral on the uppermost face is normally taken as the result. Number placement and opposite-face layouts vary among products and manufacturers. A visually pleasing layout or a particular arrangement of opposite faces does not by itself establish that a physical die is balanced.
Range and probability
A standard numbered d20 has twenty possible results. In an ideal fair model, any specific face has probability 1/20, or 5%. The average result of a uniformly numbered 1–20 die is 10.5.
Common tabletop uses
The d20 is widely used for checks, attacks, saving throws, random tables, and other resolution mechanics in games that use a d20-based system. In modern Dungeons & Dragons rules, a d20 roll is central to many tests, but that is a game-specific convention rather than a universal property of every twenty-sided die.
Natural 1 and natural 20
Tabletop players often use the phrases natural 1 and natural 20 to describe an unmodified die result of 1 or 20. Whether those values have special effects depends on the specific rule being used. The terms should not be interpreted as universal success or failure rules outside their relevant game context.
Variants and collectible d20s
D20s are produced in many materials, sizes, edge profiles, and decorative styles. Oversized d20s, countdown d20s, life counters, spindown-style number layouts, sharp-edge resin dice, metal dice, stone dice, and custom-symbol d20s may all look similar while serving different purposes. Product records should distinguish standard random-roll numbering from sequential or specialty layouts.
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