Armor penetration lets champions ignore a portion of their target's armor when dealing physical damage. There are several types of armor penetration mechanics in League of Legends:

Key Types:

• Flat Armor Reduction: Reduces armor by a fixed amount, can go below 0
• Percentage Armor Reduction: Reduces armor by a percentage, can't go below 0
• Percentage Armor Penetration: Treats target's armor as reduced by a percentage
• Lethality: Provides flat armor penetration

Order of Calculations:

1. Flat armor reduction
2. Percentage armor reduction
3. Percentage armor penetration
4. Lethality

Important Notes:

• Penetration has no effect if target's armor is 0 or less
• Percentage penetration is more effective against high-armor targets
• Lethality stacks additively while percentage penetration stacks multiplicatively
• Affects all physical damage including basic attacks and abilities
• Works on turrets but not other structures (except inhibitors)

Maximum Values:

• Highest possible lethality: 127 (160 for Aphelios)
• Highest possible armor penetration: 35% (61% for Darius)

Sources of Armor Penetration:

• Champion abilities
• Items (like Lord Dominik's Regards)
• Runes
• Monster buffs (Baron Nashor)

Special cases like Gangplank's Powder Keg and Yasuo's Last Breath have unique armor penetration interactions built into their abilities.

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