# THAC0

- Term: Tabletop term
- Canonical: https://www.lanternhall.org/lexicon/thac0/

> The number a character needed to roll on a d20 to hit a target of [Armor Class](https://www.lanternhall.org/lexicon/armor-class/) 0; subtract the target's AC to find the roll needed against anything else. It compressed pages of attack tables into one value, but its double negatives became a byword for clunky rules. It was retired when third edition introduced ascending AC in 2000.

The number a character needed to roll on a d20 to hit a target of [Armor Class](https://www.lanternhall.org/lexicon/armor-class/) 0; subtract the target's AC to find the roll needed against anything else. It compressed pages of attack tables into one value, but its double negatives became a byword for clunky rules. It was retired when third edition introduced ascending AC in 2000.

**Origin:** Implicit in the combat matrices of AD&D's 1979 Dungeon Masters Guide; it became the core attack rule of AD&D 2nd Edition (1989).

## Quick answers

**What does "THAC0" mean?**
The number a character needed to roll on a d20 to hit a target of Armor Class 0; subtract the target's AC to find the roll needed against anything else. It compressed pages of attack tables into one value, but its double negatives became a byword for clunky rules. It was retired when third edition introduced ascending AC in 2000.

**Where does the term "THAC0" come from?**
Implicit in the combat matrices of AD&D's 1979 Dungeon Masters Guide; it became the core attack rule of AD&D 2nd Edition (1989).

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