Armor class

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In some role-playing games, Armor Class (abbreviated AC) is a derived statistic which indicates how difficult it is to hit a character with an attack.

In earlier versions of Dungeons & Dragons, a lower Armor Class indicated that a creature was more difficult to hit. An unarmored human had an Armor Class 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 measured by its THAC0, meaning 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).

In third edition D&D, the armor class system was effectively reversed. An unarmored human still had an Armor Class of 10, but wearing additional armor and/or wielding a shield would instead increase Armor Class. Thus, a creature with an Armor Class of 0 in second edition would have an equivalent armor class of 20 in third edition, and vice versa. A wide range of factors affected Armor Class in both systems, including a character's physical dexterity, use of various combat techniques (such as forms of parrying), and the quality and material composition of the worn armor.

Numerous Dungeons & Dragons-derived roleplaying and computer games use the Armor Class system or a variation thereof. Many games with unrelated systems use the term to represent a character's ability to avoid damage. HackMaster, an RPG derived from AD&D, uses Armor Class in the same manner as well. Gary Gygax invented the concept of armor class as well as hit points and critical hits when he created Dungeons & Dragons.^[1]