held that game mechanics cannot be copyrighted in the United States. 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." References External links Official website of the d20 System and the Open Gaming License Open Gaming Foundation website "The Hypertext d20 SRD v3.5" – d20 System Reference Document d20Resources.com – HTML reference documentation for d20 Open Content. Jonathan Tweet games Universal role-playing games Wizards of the Coast games