The Rules Cyclopedia was a complete reference for the classic Dungeons & Dragons rules. It compiled the rules from the Basic Set, Expert Set, Companion Set, and Master Set. It is therefore a continuation of the BECMI product line, although it contains only the capsule information for Immortals from the Master Set, not the Immortal Set or the revised material from Wrath of the Immortals. This version of the game is sometimes called Basic Dungeons & Dragons, in contrast to Advanced Dungeons & Dragons; however that term is often not used to avoid confusion with the Basic Set and other introductory products themselves.
The Rules Cyclopedia was initially published in 1991 as a hardcover.
In 2003, it was made available through OneBookShelf, through their DM's Guild and DriveThruRPG portals. The original scan was replaced with a new scan. The book was made available as print-on-demand in hardcover and softcover formats.
"Appendix 1: The D&D Game World," presents the Known World. The Known World incorporates places from earlier published products, as well as the Grand Duchy of Karameikos and surrounding areas introduced in the Expert Set. Further, it describes regions of the Hollow World. Collectively, the Known World and the Hollow World would form the basis of the Mystara setting.