family, the rules for different actions, such as the first-edition hit tables or the second-edition Advanced Dungeons & Dragons (AD&D) "THAC0" and saving throw mechanics, varied considerably in which dice were used and even whether high numbers or low numbers were preferable. The d20 System is not presented as a universal system in any of its publications or free distributions, unlike game systems like GURPS. Rather, the core system has been presented in a variety of formats that have been adapted by various publishers (both Wizards of the Coast and third-party) to specific settings and genres, much like the Basic Role-Playing system common to early games by veteran role-playing game publisher Chaosium. The rules for the d20 System are defined in the System Reference Document or SRD (two separate SRDs were released, one for D&D 3rd edition and one for edition 3.5), which may be copied freely or even sold. Designed for fantasy-genre games in (usually) a pseudo-medieval setting, the SRD is drawn from the following D&D books: Player's Handbook v3.5, Expanded Psionics Handbook, Dungeon Master's Guide v3.5, Monster Manual v3.5, Deities and Demigods v3.0, Unearthed Arcana, and Epic Level Handbook. Information from these books not in the SRD include detailed descriptions, flavor text, and material Wizards of the Coast considers more specific product identity (such as references to the Greyhawk campaign setting and information on mind flayers). d20 Modern has its own SRD, called the Modern System Reference Document (MSRD). The MSRD includes material from the d20 Modern Roleplaying Game, Urban Arcana Campaign Setting, d20 Menace Manual, and d20 Future. The MSRD can cover a wide variety of genres, but is intended for a modern-day, or in the case of the last of these, a futuristic setting. History Development Ryan Dancey believed that the strength of Dungeons & Dragons was in its gaming community instead of its game system, which supported his belief in Skaff Elias's axiom known as the "Skaff Effect" which suggested that other companies only enhanced the success of the RPG market leader, which was now Wizards of the Coast. Dancey also theorized that the proliferation of game systems weakened the RPG industry, and these beliefs led to the idea to let other publishers create supplements for D&D. This led to a pair of licenses released by Wizards in 2000, prior to the release of third edition D&D: the Open Gaming License (OGL) made the D&D