Tuesday, March 29, 2011


THAC0 is an acronym for "to-hit armor class zero" in Advanced Dungeons & Dragons 2nd Edition, published in 1989. It represents the minimum target number needed by a combatant to hit or strike armour class (AC) zero on a d20 (i.e., on a 20-sided die). Against other armour classes, the target number is calculated by subtracting the opponent's AC from the combatant's THAC0. Thus, a combatant with THAC0 13 would need to roll 10+ to hit AC 3 (i.e., 13 − 3) and 18+ to hit AC −5 (i.e., 13 − [−5]).

THAC0 was a handy construct that replaced the combat matrices of the first edition game. In other words, instead of a table for each group of character classes showing the to-hit numbers by character level and AC, all THAC0 values were in a single table organized by character class group and character level. This reduced bookkeeping, for a player could simply record his character's THAC0 instead of writing out the to-hit number for each AC from 10 to −10. The DM no longer had to consult the to-hit tables, for a monster's THAC0 could be included in the stats provided by the scenario. It should be noted that THAC0 wouldn't quite work for first edition purists who used Gygax' "multiple 20" rule when the to-hit number was 20+.

Why mention THAC0 now, twenty-two years later? I invented it in 1983 (see page scans below), five years before TSR published it in FR6: Dreams of the Red Wizards, a first-edition product, and the earliest THAC0 mention of which I am aware. I called it T.H. ("to-hit"), but the concept is identical. If I believed in intellectual property, which contradicts actual (scarce) property rights, I might claim "ownership". As it is, I'm willing to settle for the honourary title of THAC0 Father—at the risk of annoying Mario Puzo's estate. On the other hand, perhaps it is inevitable that innovations which seem obvious in retrospect (retrobvious?), like THAC0, will be invented by somebody, making the idea of intellectual property ownership all the more dubious.

Above: Cover page of The Final Conflict, a typewritten adventure that uses the concept of THAC0, showing 1983 date.

Above: Page 17 of The Final Conflict, showing creature stats with T.H. (what TSR later called THAC0).

Next: "Playability vs. Realism".


  1. Fair enough. But it drives me crazy that the zero in To Hit AC Zero in your post is written with a small 'o'. Baffling. Isn't it THAC0?

    There is a frustration in being a designer for the game, and having ideas which others stumble upon and then have the money to implement. I've had a few, which I won't mention because I haven't the proof you offer above. Most of the work I did in those ages past has been lost, thrown out or written over in the computer reaches.

    You make a good case for saving everything.

  2. Aha! When I posted the comment with a numeral zero, Google automatically modified it to a small 'o' ... creepy.

  3. Alexis, Thanks for the comments. Yeah, the small 0 for a zero bugs me, too. I suppose it's the font; too lazy to look into it at the moment.