[W]e are not replacing the Basic and Expert books, we [are] attempting [to] open the game up for more opportunity to new and old players alike. You no longer have to search for those beat up copies of the rule books - it's all here and more!
The B/X retro-clone is compatible with Basic Dungeons & Dragons (specifically, the 1981 version consisting of Dungeons & Dragons Basic Set (Second Edition) and Dungeons & Dragons Expert Set) with several refinements. The revised rules are separated into Dungeon Master and Player books. Character classes, spells, weapons, monsters, and magic items have been expanded.