Talk:Tryndamere/@comment-178.41.110.48-20120625103942/@comment-24.245.106.81-20120628010141

30 armor pen is 30% more damage through armor.

You start with, generously, 80 dmg. That gets cut by 40% down to 48 dmg.

Otherwise, 60 dmg cut by only 10% is 44. Congratulations, you've traded your runes being useful all game for four additional damage against champions, and 20 more against minions for easier last hitting.

Inb4 armor pen doesn't help on skills; untrue. There is no reason not to do ArPen unless you have a very horrible early game or can't last hit, which is one of the easiest things to do in the game.