Tenacity and CC reduction
I was just wondering if anyone know's exactly how and in what order CC reduction is applied. For instance, if you played Irelia with 3 enemy champions procing her passive for 30% tenacity, merc treads for 35% tenacity, zephyr for an additional 35% tenacity and the tenacious mastery for an additional 15% tenacity how would this be applied and in what order? I know that it wouldn't just add to have 115% tenacity but the tenacious mastery states it stacks multiplicitively with other CC reduction. If anyone knows exactly how this works, I would be very grateful to know! Thanks!

I should be asking you because you are wise.

Haha wise guy eh? Well even the wise don't know everything ;)

I don't know how the all the maths work, but I do know that the tenacity from zephyr and merc treads don't stack. Those are UNIQUE passives.

Oh ok thanks! I didn't realize that, great to know!

Correct, "Tenacity" effects, such as on Zephyr and Merc Treads, do not stack. However "Crowd Control Reduction" (ccr) effects do stack both with each other and with Tenacity.
The stacking is done multiplicatively, though, not additively. Here is the math: Let's say you get hit with a standard 1 second stun. If you have Merc Treads with 35 Tenacity, you have a flat 35% ccr effect. Therefore, the stun lasts for 1 second minus 35% of 1 second. The stun lasts for 0.65 seconds. If you have the old Juggernaut mastery point from before this last patch, you would have an additional 10% ccr. This effect comes into play after the Tenacity, so you would subtract 10% of the 0.65 seconds off of the stun, resulting in a 0.585 second stun. Now let's say you're playing Irelia and there are 3 champions nearby. Your passive, Ionian Fervor, would grant you 40% ccr, so you take off 40% of the presently .585 second stun, ending with a .351 second stun. This is the duration of a 1 second stun on a champion with a total of 85% ccr. So you see, you multiply each successive percentage ccr stat by the resulting duration of the cc affect after the last ccr stat has already been applied. This way, every ccr source dilutes the effect of the next so that you never end up with ccr of 100% or more, preventing some champions, like Mundo, Irelia, and Trundle, from becoming completely immune to crowd controls. And you can even change the order in which all these sources of ccr are applied and still get the same final stun duration. It's really cool, imo. 
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