Talk:Armor penetration/@comment-99.237.170.204-20110603051725/@comment-3486816-20110608180658

nope, it means that once you did, Last whisper would have significantly less effect. Last whisper is always "active" if you will, but because it's calculated last, reducing or penetrating armor using other sources will actually lower Last Whisper's effectiveness. To illustrate, say you're facing an enemy with 150 armor and you don't have any penetration other than black cleaver and Last Whisper. Normally, Last Whisper would reduce 40% of that, and it will at first, reducing their effective armor (for your attacks only) by 60 to 90 armor. Not too shabby. However, should you start autoattacking, you'll apply black cleaver stacks which will be calculated BEFORE Last Whisper. Once you get all 3 stacks their armor will be literally reduced by 45, meaning your teammates can benefit from the reduction too. This leads to your opponent having 105 armor, and then last whisper is calculated based on that, leading to only 42 armor pen from last whisper, effectively lowering their armor to 63 for the purposes of your attacks (but still 105 for your teammates'). This makes building last whisper along with other sources of armor pen/reduction less attractive, as the more armor pen from other sources you have the less effective it is. It still is effective in those cases, just not as much, so either go with LW alone or go with black cleaver and youmuu's/brutalizer, etc.

tl;dr basically the ordering simply means it's not a good idea to build % armor pen with very many other sources of armor pen.