Talk:Armor/@comment-5139296-20120527004849/@comment-3486816-20120530030246

penetration doesn't lower armor/mr below 0, and because it's applied after reduction (which does lower armor below 0), if you've reduced someone's armor/mr to or below 0, penetration has no effect whatsoever. If you've reduced their armor/mr to near 0 such that you have more penetration (e.g. you reduce their armor to 5 and you have 10 penetration), you'll hit them as though they had 0 armor (as opposed to -5). If you, however, reduce their armor to below 0, you'll deal augmented damage as shown here in the damage multiplier function. That, along with the fact that it applies to your teammates' damage as well, is the key thing that differentiates reduction from penetration.