Armor class

In some role-playing games, armor class (abbreviated AC) is a derived statistic that indicates how difficult it is to land a successful blow on a character with an attack. It represents either a character's protective equipment, ability to dodge attacks, or a combination of the two.

In earlier versions of Dungeons & Dragons, a lower AC indicated that a creature was more difficult to hit. An unarmored human had an AC of 10, and armor lowered a character's armor class. Powerful creatures would usually have an armor class lower than 0.

In Advanced Dungeons & Dragons, a character or monster's ability to strike successfully was indicated by its THAC0, the minimum roll needed on a 20-sided die "To Hit Armor Class 0". The die roll needed to hit other armor classes could be computed by subtracting the armor class from the THAC0. The lower one's THAC0, the more likely a hit would be successful. This system replaced combat tables in the 2nd edition of AD&D, but was officially abandoned in the 3rd edition of D&D (2000).