Here's how it works. First armor reduction subtracts a flat amount from your armor. Then, if you have Last Whisper, they lose 40% of whatever armor they have left. Then, if you have any flat armor penetration, that amount is subtracted from however much armor is left. Armor reduction can reduce armor reduction to negative values, but penetration can't...and since penetration applies afterwards, it can't HELP it go below zero either.

Here's a few examples:

1) The enemy has 100 armor. I have 60 armor reduction due to the Cleaver's debuffs, Last Whisper, and 15 armor penetration from runes. Reduction takes their armor down to 40. Last Whisper reduces that to 24. Armor penetration takes it to a final value of 9.

2) The enemy has 100 armor. I don't buy the Black Cleaver, but instead have a Brutalizer (15 penetration), Last Whisper, and 15 penetration from runes. Last Whisper applies first, reducing his armor to 60. Then my 30 total armor penetration takes it down to 30. See how Last Whisper was worth 40 penetration this time, but only 16 when paired with the Black Cleaver?

3) The enemy has 50 armor. I have Black Cleaver, Last Whisper, and 15 armor penetration from runes. The Black Cleaver does 60 armor reduction, reducing their armor to -10. Last Whisper and my runes do nothing because they can't take armor below zero. If I sell Last Whisper and buy Stark's Fervor instead, though, I can get their armor down to -35.

The moral of the story is that combining armor reduction effects with Last Whisper isn't very cost-effective. You want to either combine Last Whisper with armor PENETRATION, or stack armor reduction by itself without Last Whisper, maybe with some flat penetration on top of it if you don't expect the reduction to take them to zero or below. Magic reduction/penetration/Void Staff work exactly the same, by the way.

You have a lot of information here that is just plain wrong. Look at this post in the FAQ sticky and you will see that your build really does not do what you claim it does.
http://www.leagueoflegends.com/board...ead.php?t=3287

Q: How does armor and magic resistance work? (Credit to Garek for framing it better than I could.)

Every 1 armor means the damage it takes to kill you is extended by 1% of your hitpoints. At 100 armor, it takes 100% more damage to kill you, ie it requires double damage, ie you're taking half-damage. At 300 armor you're taking 25% damage, etc.

Magic resistance works in the exact same fashion, except it does so against spells only. There is no overlap between armor and magic resistance at all. (Ex. Having 500 Armor would have no impact on spells, having 500 Magic Resistance would have no impact on physical attacks)


Q: What order is armor penetration/MR penetration applied in? (Credit to TiberiusAudley)

A: From top to bottom

Armor/MR Reduction
% Based Armor/MR Penetration
Flat/Linear Armor/MR Penetration


Q: Can you reduce armor or magic resistance below 0?

A: Yes, although the only things that can reduce below 0 is Armor/MR Reduction. Both percentage and flat penetration do not have an effect after they get to 0 armor/MR.




This means that armor penetration does not help at all if you already have used your black cleaver stacks to get an enemy to or below 0. It also means that with an enemy has 100 armor and you have black cleaver and last whisper their final armor will be (100-60)*.6=24 not 0.

Please at least know how some basic mechanics work before writing a guide full of misinformation.

@Xetian Brutalizer is armor penetration not armor reduction.

Also, a critique on your actual guide: with only 1 attack speed item it would seem like you are not getting max stacks of Black Cleaver very quickly. If you are going for an armor shredding build I would suggest something like this.

Doran's Shield
Boots
Stark's Fervor
Black Cleaver
either Trinity Force or Frozen mallet (getting both is a waste of the passive, but getting a sheen and a frozen mallet would be reasonable)
Another AS item maybe malady for more life steal

Edit: Above poster beat me too it.

I personally like a 9-21-0 build on him and rather soak dmg with him. You just go straight for the ranged carries or the spell casters and allow them to ff you while your team. I've noticed that Mord is the most anti-burst tank/carry in the game, and he's now become my main because of that. This champion will only get better.

wardens mail seems to keep the ashes and teemos at bay. at least early game. I like the sound of your armor penetration. Ive been going Dmg/AS/lifesteal and can stand toe to toe witht he nastiest of melee crit/DMG/life stealers and wreak havoc on entire groups

i will try a modified version of your AP but i will defineatly be getting lifesteal

Stark's Fervor covers both lifesteal, attackspeed and gives a hefty -25 armour aura. If you are going for armour pen, include this item.

Also, keep in mind that Mace of Spades does in fact deal magic damage, even though its based on your physical damage.

It depends on the enemies magic resistance, but it depends on your attack damage. This also means that your armor penetration would do nothing but magic penetration would help it (not necessarily worth it to get mpen tho)