Talk:Magic penetration/@comment-69.14.62.169-20111215025503/@comment-4087140-20111215054000

It means that, whatever percent magic penetration you have, Arcane Knowledge increases it by 10. If you have 0% normally, you have 10% with Arcane Knowledge. If you have 40% from Void Staff, you have 50% in combination with Arcane Knowledge. Percent magic penetration stacks additively.