Tenacity is a mechanic that reduces the duration of crowd control effects. It does not affect the potency/strength of the effect. Tenacity will not stack with other sources of Tenacity.
Tenacity works on: Blind, Forced Actions (Charm, Terrify, Fear and Taunt), Pacify, Silence, Slow and Stun.
Tenacity does not work on: Airborne (Knockback, Knockup and Pull), Attack Speed Reduction and Suppression.
Tenacity reduces the duration of Crowd Control effects by a percentage. X amount of Tenacity will grant X% reduced Crowd Control duration.
Having 35 Tenacity results in 35% reduced Crowd Control duration.
When multiple source of items have tenacity, the most powerful relevant effect applying to that statistic is the one that takes effect.
+35 tenacity
+35 tenacity
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+35 tenacity
Example: if your champion has one item granting +35 tenacity and another item granting +35 tenacity, your champion has 35 tenacity.
Crowd Control Reduction (or CC Reduction) is a mechanic that reduces the duration of crowd control effects. It does not affect the potency/strength of the effect. CC Reduction will stack multiplicatively with Tenacity and other sources of CC Reduction.
Note:Both Tenacity and CC Reduction have the exact same mechanic, but these two mechanics has different names to easily differ which stat can or cannot stack.