d20based

d20based is a term used to describe tabletop role-playing game frameworks that rely on the twenty-sided die as the central mechanism for resolving actions. In a typical d20based system, a player rolls a d20, adds ability scores, skills, and other bonuses, and compares the total to a target number such as a difficulty class, an armor class, or a saving throw threshold. The result determines success or failure, with natural 20s often yielding automatic success or critical effects and natural 1s often resulting in failure or special consequences depending on the ruleset.

History and adaptations: The most influential example is the d20 System introduced with Dungeons & Dragons 3rd edition (and continued in 3.5 and later editions), published under the Open Game License. Pathfinder, a standalone RPG that originated as a fork of D&D 3.5, uses a d20-based resolution method with its own rules. Starfinder expands the concept by mixing d20 mechanics with science-fantasy elements. Dozens of indie games and house rules describe d20based resolution in combat, skills, or social interactions.

Design considerations: Advantages of d20based design include broad player familiarity, a single dice type underpinning many modules, and straightforward probability curves that emphasize average outcomes. Critics note potential predictability and dependence on modifiers, with the distribution biased toward mid-range results unless counterbalanced by advantages, disadvantages, or alternative mechanics.

Conclusion: As a descriptive category, d20based encapsulates a wide range of games that share the essential roll-a-d20 mechanic, even as individual games diverge in detail, such as action economy, proficiency systems, and critical success rules.