Hey forum. I was just wondering today when you build magic pen and when you build AP on your casters. I've been just kinda treating them as one and the same and that seems to work at low level play, but I know there's more to it. Obviously if the enemy team is stacking magic resist you want to build pen, but isn't magic pen good against people without magic resist too? Is there a hard, fast rule about it? Do the same rules apply to AD vs. armor pen? People don't seem to build armor pen as much as they do magic pen...