How does arm pen stack?

[quote=TheKut]If I'm for example playing [[Darius]] with a maxed [[Apprehend]] with 25% arm pen and have a [[Last Whisper]], does this stack for a total of 65% or is it something else? And if I have flat arm pen, like [[The Brutalizer]] with [[Last Whisper]], does it remove 20 armor for TB first and then 40% of what's left or the opposite?[/quote]

[quote=Luther3000]It stacks multiplicatively (basically less) and percentage pen is applied after flat.[/quote]

[quote=lifebaka]Maxed Apprehend and a Last Whisper give a total of 55% armor penetration. Add in the 10% mastery and you're at 59.6% armor penetration. This is because percentage armor penetration stacks multiplicatively. To figure out the total, you have to multiply their effects on enemy armor together. Apprehend gives 25% penetration, so it's the same as a 75% armor multiplier. Last Whisper gives 40% penetration, so it's the same as a 60% armor multiplier. 75% * 60% = 45% armor multiplier, which is the same as 55% armor penetration.[/quote]

[quote=caucheka]and to answer your other question, yes flat arpen is calculated before %, therefore making it a bad idea to get both flat and %.[/quote]

[quote=IceCreamy][[Armor Penetration]][/quote]

[quote=Pølsemanden]But now i ask you a question of great importance. DOES BRUTALISER N YUMOUS STACK? DUNDUNDUN Armor pen reds n Quints+brutaliser+yumous=60 arpen, WUUUT - Now throw a TBC in there - DUN DUN DUN DUUUUUN True-damage, we haz it.[/quote]

[quote=lifebaka]True damage... Against anyone who hasn't built any armor. Against people who have built armor (which is, like, everyone), you don't do true damage, don't have enough damage to make the fact that you're ignoring virtually all of their armor worth while, and are extremely squishy. Maybe you can use it to stomp on some noobs, but I wouldn't suggest it in a serious game. (Also, if you're going to do this, build it on something like bruiser [[Taric]] for additional flat armor reduction. -135 armor, anyone?) But yeah, they stack. UNIQUE Passives from different items always stack unless they explicitly say otherwise.[/quote]

[quote=Pølsemanden]I'm thinking about shaco, since his Q scales with ad n armorpen, this could maybe be pretty strong on him, or maybe it's just dumb. I tried it in a game, 3 shot a ashe. i was kinda fed though :p 2 normal hits, 1 q and then shiv. GG[/quote]