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The d20 System is a role-playing game system published in 2000 by Wizards of the Coast originally developed for the third edition of Dungeons & Dragons. The system is named after the iconic 20-sided dice which are central to the core mechanics of many actions in the game.
Much of the d20 System was released as the System Reference Document (SRD) under the Open Game License (OGL) as Open Game Content (OGC), which allows commercial and non-commercial publishers to release modifications or supplements to the system without paying for the use of the system's associated intellectual property, which is owned by Wizards of the Coast.
The original impetus for the open licensing of the d20 System was the economics of producing roleplaying games. Game supplements suffered far more diminished sales over time than the core books required to play the game. Ryan Dancey, Dungeons and Dragons' brand manager at the time, directed the effort of licensing the new edition of Dungeons and Dragons through the 'd20 System Trademark', allowing other companies to support the d20 System under a common brand identity. This is distinct from the Open Game License, which simply allows any party to produce works composed or derivative of designated Open Game Content.
The d20 System is a derivative of the third edition D&D game system. The three primary designers behind the d20 System were Jonathan Tweet, Monte Cook and Skip Williams; many others contributed, most notably Richard Baker and Wizards of the Coast then-president Peter Adkison. Many give Tweet the bulk of the credit for the basic resolution mechanic, citing similarities to the system behind his game Ars Magica. Tweet, however, stated "The other designers already had a core mechanic similar to the current one when I joined the design team".
To resolve an action in the d20 System, a player rolls a 20-sided die and adds modifiers based on the natural aptitude of the character (defined by six abilities: Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma) and how skilled the character is in various fields (such as in combat), as well as other, situational modifiers. If the result is greater than or equal to a target number (called a Difficulty Class or DC) then the action succeeds. This is called the Core Mechanic. This system is consistently used for all action resolution in the d20 System: in prior games in the D&D family, the rules for different actions, such as the first-edition hit tables or the second-edition 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 games 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 RPG 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 edition 3.0 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 Dungeons & Dragons 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 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, the d20 Menace Manual, and d20 Future; this 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.
Trademark license 
Because Dungeons & Dragons is the most popular role-playing game in the world, many third party publishers of the 2000s produced products designed to be compatible with that game and its cousin, d20 Modern. Wizards of the Coast provided a separate license allowing publishers to use some of its trademarked terms and a distinctive logo to help consumers identify these products. This was known as the d20 System Trademark License. The d20 System Trademark License (D20STL) required publishers to exclude character creation and advancement rules, apply certain notices and adhere to an acceptable content policy. D20STL products were also required to clearly state that they require the core books from Wizards of the Coast for use. All D20STL products also had to use the OGL to make use of d20 System open content, but publishers were able to use the OGL without using the D20STL. Games that only use the OGL were (and are) not bound by these restrictions, and several have included character creation and advancement rules, allowing them to be used as standalone products.
With the release of the Fourth Edition of Dungeons & Dragons in 2008, Wizards of the Coast revoked the original D20STL, replacing it with a new license specifically for D&D, known as the Game System License. The terms of this license are similar to the D20STL, but there is no associated OGL or Open Content, and the fourth edition SRD merely lists the items and terms which may be used in licensed products. Of course, third edition/D20-based products may still be released under the OGL alone.
||This article's Criticism or Controversy section may compromise the article's neutral point of view of the subject. (May 2011)|
Unlike the OGL, the d20 System Trademark License (D20STL) is revocable and is controlled by Wizards of the Coast. Wizards of the Coast has the ability to alter the d20 System Trademark License at will and gives a short, 30 day "cure period" to rectify any issues with the license before termination. These changes apply retroactively to all material published under the d20 System Trademark License.
When gaming company The Valar Project, under former Wizards of the Coast brand manager Anthony Valtera, attempted to publish the d20 Book of Erotic Fantasy (BoEF), which focused on sexual content, Wizards of the Coast altered the d20 System Trademark License in advance of publication of BoEF by adding a "quality standards" provision that required publishers comply with "community standards of decency." This subsequently prevented the book's publication under the D20STL. Wizards of the Coast said this was done to protect its d20 System trademark. The Book of Erotic Fantasy was subsequently published without the d20 System trademark under the OGL. Other books subsequently published under similar circumstances include Skirmisher Publishing LLC's Nuisances which also includes on its cover the disclaimer "Warning: Intended For Mature Readers Only."
The same round of changes to the license also limited the size at which the text "Requires the use of the Dungeons & Dragons Player's Handbook, Third Edition, published by Wizards of the Coast" (which is required to appear on the front or back cover of most fantasy d20 System products) could be printed, and prohibited making part of it larger than the rest. This was perceived as being aimed at the same Valar book; early mockups of the cover had the words "Dungeons & Dragons" in the above text printed much larger and in a different font from the rest, right at the top of the front cover. This could have made the book appear to be an official Dungeons & Dragons publication to a casual or uninformed observer. The published version does not have the offending text on the cover.
Criticism is also levied at the conditions for termination of the d20 System Trademark License through a breach of its terms. The license requires that, upon breach of the terms of the D20STL which includes any subsequent modifications of the license after publication of a work using the d20 System trademark, all inventory and marketing material must be destroyed. Adhering to the breach conditions is an onerous task for smaller game companies. The mere threat of this condition being imposed was a huge blow to the now defunct d20 System publisher Fast Forward Entertainment, which had released several books that used non-open Wizards of the Coast content due to company president James Ward's misunderstanding of the license.
Other criticism is based around the part of the d20 System Trademark License which defines "Open Game Content" to include game mechanics and purports to license it. It is widely believed that game mechanics are uncopyrightable in the USA, and according to a circular on the US Copyright Office's website, "Once a game has been made public, nothing in the copyright law prevents others from developing another game based on similar principles."
One result of this has been the abandonment of the d20 System License by some publishers in favor of a simple "OGL" designation. Mongoose Publishing's licensed games based on the Conan the Barbarian property and the Robert A. Heinlein novel Starship Troopers, for example, use systems that function nearly identically to d20 but do not carry the d20 logo.
- The d20 System Concept: Frequently Asked Questions
- Dungeons, Dragons, and d20 - An Interview with Jonathan Tweet by Therese Littleton, Amazon.com
- System Resource Document - The Basics And Ability Scores
- System Reference Document - Frequently Asked Questions
- Waters, Darren (2004-04-26). "What happened to Dungeons and Dragons?". BBC News Online (BBC). Retrieved 2009-06-02.
- Svensson, Peter (2008-07-21). "Dungeons & Dragons reborn". Associated Press (Associated Newspapers). Archived from the original on 2009-03-21. Retrieved 2009-06-02.
- "Fourth Edition GSL FAQ". Wizards of the Coast. Retrieved Oct 3, 2012.
- Book of Erotic Fantasy Loses D20 license
- US Copyright Office
- Official website of the d20 System and the Open Gaming License
- THE (UNAUTHORIZED UNOFFICIAL) OPEN GAMING LICENSE OGL D20 FAQ - Comprehensive, Unbiased D20 History (1/1/2009)
- Open Gaming Foundation
- The Hypertext d20 SRD – v3.5 d20 System Reference Document
- D20 System at the Open Directory Project
- d20 Resources - HTML reference documentation for d20 Open Content.
- D20 System Game