d20 System

The twenty-sided die, or d20, is practically synonymous with tabletop roleplaying, going all the way back to its use in the original release of Dungeons & Dragons in 1974. But the d20 system we know today didn't actually come about until D&D 3rd edition, after Wizards of the Coast bought the rights to the game from its original publisher TSR. This edition standardized the use of the d20 into a core mechanic, something D&D didn't really have before. While some rules used the d20 in earlier editions, there was no standardized mechanic for resolving everything; rather each situation had its own dice, tables, and procedures, which may or may not have included a d20. The core mechanic of the d20 system, as introduced in 2000 with D&D 3E, involves rolling a 20-sided die, adding any bonuses, penalties, or other modifiers, and comparing it to a target number, sometimes called a Difficulty Class or DC. If the result of the roll is equal to or higher than the DC, the roll is a success. If it's lower, the roll is a failure. This determines the result of the rolling character's attempt. Bonuses come from a character's stats, skills, abilities, and equipment, and additional bonuses or penalties may arise from their specific circumstances and opposition. This mechanic resolves the use of skills, attacking other creatures, knowledge checks to determine what a character knows or remembers, and more. A roll of 20 on the die, or "natural 20," is the best possible result, and depending on the game, it might be a critical hit, an automatic success, or simply the best your character could have possibly done. A roll of 1, or "natural 1," might be an automatic failure, the worst possible outcome, or a devastating fumble. Wizards of the Coast created the d20 system as a "standardized" system for roleplaying games, and placed on an open license so that other games could use it as well, making them broadly compatible. D&D 3E's updated system 3.5, as well as its spiritual successor Pathfinder used this system, as did plenty of other games, including Mutants and Masterminds, Spycraft, 13th Age, Big Eyes Small Mouth, and Conan: The Roleplaying Game. Technically, after Wizards produced D&D 4th Edition in 2008, they were no longer using the d20 system. However, the core mechanic was essentially the same, and most people today use the term "d20 system" not to refer to the specific generic ruleset of D&D 3E and 3.5, but to any roleplaying game that uses the "d20 plus modifiers compared to target number" as its core mechanic. Under that definition, D&D 4th and 5th editions, Pathfinder 2E, Cypher System, Shadowdark, Stars/Worlds/Cities Without Number, Quest, Lancer, Shadow of the Demon Lord, and many, many more use a modified version of the d20 system. To stretch the definition even further, some people might use "d20 system" to describe any RPG that uses a d20 in its core mechanic's resolution, opening the door to "d20 roll-under" systems where players want to roll low on the d20, under their stat or target number to succeed, or games that otherwise roll a d20 but don't add to or modify it, like Mörk Borg and its variants, Perils & Princesses, Cairn, Mausritter, Into the Odd, and many other games.