Thac

Thac is an abbreviation used in role‑playing game design, most notably in early editions of Dungeons & Dragons, to refer to the body of rules governing how a character or monster makes an attack roll to overcome a target’s armor class (AC). The full form of the abbreviation is “To Hit Armor Class,” and in the first two editions this convenience was usually written as THAC0 (to hit AC 0). The concept was devised in 1977 for the first edition of Advanced Dungeons & Dragons (AD&D) as a quick method for determining whether an attack would hit a target whose AC was expressed relative to 0. In that system a character’s THAC0 value was a single number, and the player would subtract the target’s AC from that value; a result equal to or less than the roll on a twenty‑sided die meant a successful hit.

For example, if a fighter’s THAC0 was 15 and the target’s AC was –5 (which was common for heavily armored characters), the fighter would need to roll a 20 (15 – (–5) = 20). As characters gained levels, their THAC0 values decreased, reflecting greater skill or weapon proficiency. Monster entries in the Monster Manual also carried a THAC0 number. Explicit tables in the Player’s Handbook and Monster Manual allowed players and game masters to calculate hit probabilities with a simple arithmetic substitution.

The THAC0 system was popular for over a decade, but in 3rd edition of D&D the mathematics of fighting were redesigned to make AC values increasing with difficulty and to obviate the need for tables. Modern editions use a straightforward “attack bonus” system; nonetheless, the term “thac” remains a common nickname among long‑time players and game designers, hinting at the legacy of the THAC0 system in the early history of tabletop role‑playing games.