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.
The origins of THAC0 have been the subject of considerable curiosity. It is unsurprising that the game Dungeons & Dragons shipped without the concept of THAC0: in the original three booklets, armor class ranged from nine to two, so zero was out of the question. When Greyhawk arrived in 1975, it brought with it a handful of monsters that bucked this trend: Tiamat, in her first appearance in games, had an armor class of zero, and the newly-introduced Will O'Wisp sported a shocking armor class of negative eight. As new monsters joined the bestiary in later expansions, zero settled into the middle of the armor class scale.
Los Angeles attracted a vibrant community of early adopters of Dungeons & Dragons, many of them students at local universities; it is unsurprising to find the roots of THAC0 in the UCLA Computer Club. In issue #32 of Alarums & Excursions, Sheldon Linker helpfully clarified the UCLA implementation of THAC0.
Linker does acknowledge that calculating THAC0 for original Dungeons & Dragons depends on many factors of a character, and that "THAC0 must be recomputed when any of this changes." The difficulty of recomputing THAC0 surely provided the biggest obstacle to its immediate adoption; the later success of THAC0 in second edition depended heavily on defining it in a way that only infrequently necessitated recalculation.
Los Angeles pioneers were not the only ones eager to speed up play by computing preliminary values before combat. The need for quick calculations was especially pressing for early tournament play, where games were typically time-constrained. While we don't see any evidence of such optimizations in the 1975 version of the Tomb of Horrors, by the following year, when Gary Gygax designed the original version of the Lost Caverns of Tsojconth (later module S4) for the Detroit Wintercon tournament dungeon, he had introduced a very interesting pre-calculated combat system.
Tsojconth was designed for six pre-generated characters. This excerpt from the dungeon master's cheat sheet assigns an "unadjusted die roll to hit" as a defensive characteristic to each of those six numbered characters in the upper right hand corner. On the offensive side, every monster had a "hit bonus," and to determine whether or not a monster hits with an attack, one simply adds that hit bonus to a d20 roll and determines if it exceeds the "unadjusted die roll to hit" for the target character. That latter figure was in turn derived by transforming the character's armor class, dexterity bonus, and relevant magic items into an additive defensive statistic. Similarly, the hit bonus aggregates all of the attacker's offensive bonuses into one sum. Thus, in order to hit pre-generated character #2 (Ethelrede the Fighter, with a defensive stat of 23), a Bronze Dragon (with a hit bonus of 9) needed to roll a 14 or higher. A separate sheet included with the tournament module similarly assigns a hit bonus to the characters and defensive totals to monsters. Readers may find this 1976 Gygax optimization eerily similar to third edition Dungeons & Dragons combat, as if he had leapfrogged THAC0 entirely.
But Gygax apparently did not think this optimization belonged in first edition Advanced Dungeons & Dragons. Instead, we find in Appendix E of the Dungeon Masters Guide (1979) a different sort of dungeon master cheat sheet for monster attacks with a column listing values needed "To Hit A.C. 0." This is usually taken as the first real use of the concept of THAC0 in official Dungeons & Dragons material, even if it does not consolidate the acronym. But note that calculating THAC0 for monsters in first edition AD&D is simpler than it is for players, given that monsters do not go up in level. Characters required something more complicated, as THAC0 would depend on weapons and other factors: the closest that character sheets of the era get to recording THAC0 is a rather more complex chart for pre-computed "to hit armor class" ranges of values - one that would still require frequent updates.
Even though Advanced Dungeons & Dragons did not initially embrace THAC0, the fan community kept it alive. In Alarums #60 (July 1980), we can find Scott Turner describing a variant he uses with "no big charts, no messy tables or calculations, just one simple addition: Base to Hit = THAC0 - AC," where he envisions a standard initial THAC0 of 20. Proposals like these kept THAC0 in the public eye, and eventually, TSR elevated THAC0 to a linchpin of the Dungeons & Dragons system... at least until third edition came along, though perhaps even its combat system has its prefigurement in the extraordinary creative energy of the 1970s.
Nowhere is that energy on better display than in the pages of Alarums & Excursions. Incidentally, this year marks the fortieth anniversary of the first publication of Alarums & Excursions. It is nearly impossible to overestimate the number of innovations that debuted in its pages; for example, I have previously discussed the appearance of dice notation in its very first issue. Amazingly, Alarums & Excursions continues to this day, still under the benevolent oversight of Lee Gold.
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