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 (see THAC0). An unarmored human had an Armor Class of 10, and armor lowered a character's armor class. A very powerful creature usually had an Armor Class lower than 0. In third edition D&D, the armor class system has been effectively reversed. An unarmored human still has an Armor Class of 10, but wearing additional armor and/or wielding a shield, now increases 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, a current RPG derived from AD&D, uses Armor Class in same manner as well.
