Talk:Armor penetration/@comment-60.48.60.197-20110528102525/@comment-3486816-20110608181054

Penetration only affects your attacks and cannot reduce your opponent's armor below 0. Reduction benefits all physical damage on the target (even that of minions and allied champions) and CAN reduce your opponent's armor below 0 (but only by itself, because of the order in which they're applied, you need more reduction than the opponent has armor to reduce it below 0, as armor pen doesn't help with that cause at all).