"BX" D&D refers to the combination of the 1981 Tom Moldvay Basic rulebook (the red book in the magenta box) and the 1981 David Cook/Steve Marsh Expert rulebook (the blue book). X is used to represent the Expert rules rather than E because the modules of that era used an X for Expert; also, this helps separate the game from the later Frank Mentzer BE, BECM, or BECMI game.
Use this tag to indicate interest specifically in the B/X version of D&D. Use Classic D&D to indicate broader interest in any version of original or basic D&D, including B/X. Use Basic D&D to indicate interest in any version of Classic D&D other than OD&D.