User blog comment:Sjjtitan2019/Itemization and Build Paths/@comment-12314620-20140108221703

I think you're not quite aware how Armor penetration works... Armor penetration and armor reduction aren't same thing. Too lazy to explain it now (you can find good article about it on this wiki) but basically LW > 5 stack Black Cleaver if your opponent has more than 100 armor, though you should account for the fact that Black Cleaver also amplifies your teams other physical damage dealers, though if you really need it's very often better item for a bruiser to get (unless you are Miss Fortune with heavy physical AoE dealers who can in best case scenario apply max stacks on multiple targets with a single ult.