THAC0

THAC0 (often pronounced THA-ko) is an acronym constructed from the phrase "To Hit Armor Class 0" which comes from the tabletop role-playing game Dungeons & Dragons. THAC0 was used in second edition D&D and earlier. In the third edition of D&D the armor class system was changed and THAC0 is no longer used.

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.

To make an attack, the player rolls a 20-sided die. If the number equals or exceeds the attacking creature's THAC0, the attacking creature 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 the creature is poor at attacking, while low THAC0s represent higher skill at attacking. 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 unintuitive. 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. However, the 3rd Edition critical hit system tends to make armor class itself a redundant concept; theoretically, a dagger could inflict a deadly wound on a huge creature with thick hide or heavy armor.