The D20 System Trademark License

From the introduction: You are hereby granted the non-transferable, non-exclusive, non-sublicensable, royalty-free license to use the d20 System trademark logos, the d20 System trademark, and certain other trademarks and copyrights owned by Wizards of the Coast (the "Licensed Articles") in accordance with the conditions specified in the current version of this License and the d20 System Guide. User summary: The formal, legal license used by 3rd party publishers to produce compatible products.