Arpen/reduction works by minimizing the efficiency of stacking multiple kinds of it. It applies in the following order (roughly):
* Armor reductions and shreds (Black Cleaver) < Can reduce armor below 0
* Flat Armor penetration < cannot force effective armor below 0
* Percentage armor penetration < cannot force effective armor below 0
Applying Black Cleaver stacks then a Last Whisper results in the black cleaver reductions only ending up giving 27 (or 9 per stack) total.
* Armor reductions and shreds (Black Cleaver) < Can reduce armor below 0
* Flat Armor penetration < cannot force effective armor below 0
* Percentage armor penetration < cannot force effective armor below 0
Applying Black Cleaver stacks then a Last Whisper results in the black cleaver reductions only ending up giving 27 (or 9 per stack) total.
Nasus says hi

Like RCIX said, it reduces it's effectiveness.
It goes in this order:
Percent Armor Reduction:
Dragon Strike,
Slice and Dice
Flat Armor Reduction:
Black Cleaver
Flat Armor Penetration:
Youmuu's Ghostblade,
Ragnarok
Percentage Armor Penetration:
Last Whisper
Armor penetration can not take an enemies armor below 0, where as reduction can.
If you were to stack
Black Cleaver and
Last Whisper, against a 200 armor opponent, you would reduce 45 armor (
Black Cleaver), and then penetrate 62 armor (
Last Whisper), which is 107 armor gotten rid of. As opposed to 80 armor penetrated from
Last Whisper alone. It just makes
Last Whisper less effective paired together with
Black Cleaver, and usually makes the combo more cost inefficient.
It goes in this order:
Percent Armor Reduction:


Flat Armor Reduction:

Flat Armor Penetration:


Percentage Armor Penetration:

Armor penetration can not take an enemies armor below 0, where as reduction can.
If you were to stack








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I win:
Armor Penetration

DuffTime wrote:
ok ok plz carry me omg
i was only waiting for you to ask
Temzilla wrote:
Too hot to be icecream.
Luther3000 wrote:
He looks like a hair gel advert on legs
Toshabi wrote:
Icecreamy, with hair as slick and smooth as the ocean waves of Cocobana
Well that is where the wiki is for right? ;D
DuffTime wrote:
ok ok plz carry me omg
i was only waiting for you to ask
Temzilla wrote:
Too hot to be icecream.
Luther3000 wrote:
He looks like a hair gel advert on legs
Toshabi wrote:
Icecreamy, with hair as slick and smooth as the ocean waves of Cocobana
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Not sure if there was already a post about this but please answer!!:D