I know the title is a little iffy sounding but currently Lethality shows how much Armor it's negating Serrated dirk offers 15 Lethality and in the tooltip, it shows it's negating 15-18 (I don't remember the exact number) Armor.
I think this would be an awesome Idea for a magic Item, like 
to show how much magic resist it's negating.
Sp33d Zer07 votes+15 magic penetration. That means you penetrate 15 magic resist.
TryMucinex8 votesSeems complex, please elaborate.
Sp33d Zer02 votesOkay, so these boots were one owned by a great sorcerer from Demacia. This sorcerer was a genius on the battlefield who was an expert at targeting the weak spot in an enemy's armor in order to maximize the destructive force of his magic. Unfortunately, rito decided to make his scaling absolute shit, but gave him good base damage. Because of this, he decided to imbue his boots with 15 magic penetration; that way he could easily abuse those high base numbers without having to drop a ludicrous amount of gold on his neighbor's stupid looking hat.
KaRniVooL131 votes"stupid looking hat"
LMAO
[slayer-pantheon-thumbs]
RiotMeddler14 votes15=15
Sp33d Zer05 votesI feel like I just got dodged by a rioter.
Bronze Demon1 votes15=15
what about % pen items
M00ndanc31 votespercent pen would not be shown on your character display since each enemy would have a different amount of armor to cleave a percent of of.
but it could be possible if maybe items like last wisper(and its final builds) along with void staff to tell you at the bottom how much it negates from each champion in a simular way locket says how much it has shielded
LethalTempo Jhin4 votesDude.
15 Magic penetration means you ignore 15 MR.
There is no scaling on that, unlike 
items.
doodlebob6661 votesmagic resist is always reducing mr by the amount it says it never scales,
ModThe Djinn1 votesAh. The issue here is that Magic Pen and Lethality are different mechanics that SEEM like they should be the same. It's a little unclear, so no worries. :)
Magic Penetration just gives you the face value at all times. If it says 15, it ignores 15 points of magic resistance.
Lethality actually works on the following formula:
Listed Lethality Value × (0.6 + 0.4 × your level ÷ 18)
As a result it's exact value varies based on your current level.
