THAC0 (pronounced THA-ko or THAK-oh /'θæ.kɔʊ) is an acronym constructed from the phrase "To Hit Armor Class 0" which comes from the tabletop role-playing game Dungeons & Dragons. THAC0 was introduced in basic Dungeon & Dragon (1985 edition) and continued in use in the 2nd edition of Advanced Dungeon & Dragons (1989). In the 3rd edition of D&D (2000), the armor class system was changed and THAC0 is no longer used in the following D&D editions. Thac0 is still sometimes unofficially used with the Hackmaster RPG, which is derived from 1st and 2nd edition AD&D.
THAC0 is a convenient way to keep track of the ability of a character or monster to successfully strike another character or monster in combat. Each creature has an armor class that determines how hard it is to hit that creature, with lower numbers representing higher difficulty. For example, a normal, unarmored human has an armor class of 10 while a heavily armored fighter might have an armor class of 1 or 0.
A point which can be confusing to players who picked up D&D as of 3rd edition is that the acronym THAC0 seems to be something that the defender applies, since it contains "AC" (Armor Class). Instead, THAC0 is a measure of the attacker's ability to hit a specified armor class, and thus belongs to the attacker. The following description will make sense to a 3rd edition player when this is kept in mind.
The success of a character's or monster's attack is determined by rolling a 20-sided die. If the number obtained equals or exceeds the attacker's THAC0, the attacker has successfully hit a target with armor class 0. If the target has an armor class different from zero, the target's armor class is subtracted from the attacker's THAC0, and that number is what the attacker's roll must equal or exceed.
High THAC0s signify that an attacker is poor at hitting targets, while low THAC0s indicate the attacker can hit targets with ease. There is nothing particularly significant about an armor class of 0 other than the fact that it simplifies calculations.
The idea that a creature is harder to hit when its armor class goes down was felt by some players to be counterintuitive. While some players mourned the loss of the THAC0 system in the change to 3rd Edition D&D, many invited the new system, which features a larger armor class rating as it becomes harder to hit a creature and some more flexible ways to alter armor class.
The main purpose of THAC0 was as a balancing agent. In the system it was impossible to get a THAC0 lower than 3, or for a character's armor score (AC) to go lower than -10. Characters could have multiple bonuses to their attack roll but cumulatively no higher than 9. This leaves a possibility that the most powerful characters could still have a chance of missing a well armored opponent. (3-9=(-6) -(1d20) = (-7 to -26) in this instance the attacking character would have a 3 in 20 chance to miss the armored opponent. This built-in assurance of the possibility of missing was dropped in 3rd Edition in order to not limit characters upper Armor Class or Attack Bonus. THAC0 was also intended to replace the combat tables that were used in the 1st edition of AD&D.
