System Reference Document
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In the open gaming movement, a System Reference Document (SRD) is reference for a role-playing game's mechanics licensed under the Open Game License (OGL) to allow other publishers to make material compatible with that game.
The first SRD was published in 2000 by Wizards of the Coast (WotC) and is based on the third edition of Dungeons and Dragons; it was revised following the release of version 3.5 of D&D in 2003. That SRD allowed for third-party publishers to freely produce material compatible with D&D. It also formed the basis for independent role-playing games from other publishers, such as Mutants & Masterminds and the Pathfinder Roleplaying Game, among others.
The 4th edition of D&D, released in 2008, was not licensed under the OGL, but under the more restrictive Game System License. Subsequently, the 4e System Reference Document is quite different. Instead of the full texts of the OGL-licensed rules, the 4e SRD presents only lists of concepts and tables from the 4e rulebooks that may be used in a compatible product.
The 5th edition of D&D was released in 2014. A new OGL-licensed SRD based on 5th edition was released in January 2016.
Some other game systems, such as FATE, the Mongoose Publishing editions of RuneQuest, and Zweihänder Grim & Perilous RPG have also released their own mechanics under distinct OGL-licensed "System Reference Documents".