The die roll value required for an attack to hit an armor class of zero, or "THAC0," is the signature combat mechanism of the second edition of Advanced Dungeons & Dragons. Revered by some and reviled by others, THAC0 replaced the combat matrices of first edition AD&D with pre-calculated values intended to be faster and more intuitive. Astute observers have long noted foreshadowing of a THAC0 system sprinkled throughout some first edition AD&D texts. It is however less widely known that THAC0 was in use with the original Dungeons & Dragons game, prior to the publication of the Players Handbook or Dungeon Masters Guide. The excerpt above is from Alarums & Excursions #31 (February 1978), and it describes the contemporary use of THAC0, including the acronym itself.
