THAC0 eventually leaked out into published books, though Kask could not recall which writer originally invented THAC0, or who first used it in a sourcebook. In 2017, former TSR employee Tim Kask described THAC0 as originally created as an in-house design tool to calculate average hit chances. A goal of tournament adventure module design at this time was to standardize the D&D game experience to allow fair play of competetive modules. In 2020, Jeff Grubb described THAC0 as an invention of the convention tournament scene, used to convey information to DMs. Similar tables for player characters and monsters appeared in the AD&D Dungeon Masters Guide (1e) (1979), p.74-75 and the various Basic/Expert rules sets. hit dice, a concept roughly equivalent to character level. A separate chart on page 20 determined monster to-hit values based on their 'dice', i.e. In the original D&D rulebook Men & Magic (1974), p.190, a chart was provided to determine the roll needed to hit, based on the attacker's level and the target's armor class.

In 2020, former TSR employee Jeff Grubb favored the pronunciation 'thay-ko'. In a YouTube video in 2017, former TSR employee Tim Kask preferred the pronunciation 'thay-ko'. THAC0 is variously prounounced 'thack-oh' or 'thay-ko'. Against an opponent with AC6, he hits on a roll between 8 and 20 (14 − 6 = 8). įor example, a character with a THAC0 of 14 must roll between 14 and 20 on a d20 to hit an opponent with AC 0. The target's armor class is subtracted from the attacker's THAC0 when determining a hit. In practice, most opponents do not have an armor class of zero.

Against an opponent with an armor class of zero, you must roll equal to or above your own THAC0 on a twenty-sided die in order to score a hit. Prior to the publication of D&D 3rd edition in the year 2000, Dungeons & Dragons used a descending armor class scale, where a lower armor class meant that the character was harder to hit.Įach player character and monster has a statistic known as THAC0, which is calculated based on their level or hit dice.

Dungeons & Dragons by Jeff Grubb