THAC0

THAC0 is a rule used in Advanced Dungeons & Dragons 2nd Edition, to form a baseline by which a character's attack roll is calculated.

The term THAC0 is an acronym for "To Hit Armor Class 0," and represents the lowest unmodified attack roll a character may make on a 20-sided icosahedral die and still hit a target with an [Armor Class] of 0.

In gameplay, a character's THAC0 forms the baseline for all attack rolls, and that number is then modified by a number of permanent or situational variables, such as the character's Strength and Dexterity Ability Scores, the plus bonus of any magic weapons used, and a target's Armor Class.

Regardless of a character's THAC0 and all modifiers, in Second Edition Dungeons & Dragons a roll of 20 is always a critical hit, and a 1 is always a critical miss.

AD&D 2nd Edition has four different THAC0 progressions, based on rate of decrease and by level. In Dungeons & Dragons 3rd Edition and Edition 3.5, Armor Class was reworked to always trend upward instead of having a baseline of 0, with superior scores running into negative integers (for example, a [Red Dragon] whose Armor Class was once -10 might now have an Armor Class of 30 or higher).. At the same time, THAC0 was replaced by a roughly equivalent attack bonus rating based on class and level.
 * Warrior classes ([Barbarian], [Fighter], [Paladin], [Ranger], and [Monk]) have the best THAC0 progression, with a THAC0 that decreases at a rate of 1:1.
 * Priest classes ([Cleric], [Druid]), had the second best THACO progression in the game at a rate of 2:3.
 * Rogue classes ([Thief], [Bard]) had a THAC0 progression of 1:2, giving them the mathematical oddity of having a better THAC0 than a priest at level 3. Rogues remain roughly on-par with priests (albeit slightly in the Priest's favor) until level 9, and are thereafter surpassed by priest classes at an increasing pace.
 * Wizard classes ([Mage]) have the worst THAC0 progression, at a rate of 1:3.