Talk:Armor penetration/@comment-79.41.54.69-20110429162825/@comment-129.123.122.88-20110429194345

yes, flat armor reduction allows you to make your target have negative armor. For example, 3 Black Cleaver stacks on a Champ with 40 armor will make him have -5 armor and take 5/105 = ~4.75% more damage from physical attacks. (same as armor reduction formula, but increase instead of reduction).

The ONLY way to get a target to negative armor is Flat Armor Reduction - Stark's Fervor Aura or Black Cleaver stacks. Percent Armor Reduction will not reduce a target below 0 armor, and neither Percent nor Flat Armor Penetration will reduce a target below 0 armor.