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Armor Penetration is a value often found on Leather and Mail gear that gives the player the ability to "pierce" a percentage of the target's Armor. Armor Penetration Rating converts into Armor Penetration Points at the rate of about 7:1. More important, though, is the amount of Armor Penetration Rating required to ignore 1% of a target's armor, and that value is 12.31:1. In other words, 12.31 Armor Penetration Rating directly converts into 1% of ignored armor at level 80. Of course, Armor Penetration is only useful for physical damage classes like Rogues, Hunters, DPS Warriors, Death Knights, Paladins, Feral Druids, and Enhancement Shamans. It is
less useful for Frost and Unholy Death Knights because much of their damage is magic based, whereas Blood Death Knights need ArP because they're mostly physical damage. Armor Penetration is found on red gems.