So Irelia can technically get 100% tenacity right?

The tenacities aren't additive, I don't think. Some are multiplicative to others, I believe.

You can't reach 100% but you can about 70-80%. You will notice that for example Morgana stun won't last whole day it will be less than 0,5 seconds.

Or unless it caps off at a certain point. I don't remember .

Hmm interesting. So does her Passive effectively give her 70% when outnumbered with Merc treads or just 40%?

they stack multiplicatively. so the higher you go the harder the cap gets. its a soft cap basically. that increases very little as you go higher depending on how much more your adding. basically consider the most you can possibly get is like 50-60%.

Nope.

Tenacity stacks multiplicatively, not additively.

They stack multiplicative so you would get (1-0.4)(1-0.3)(1-0.15)*(1-0.15) = 0.30345 cc duration which converted to tenacity is 1 - 0.30345 = 0.69655% tenacity

.7 x .75 x .85 (Mastery) x .6 = 73.225% reduction

A 3.25 second root would last .87 seconds.

~.9 seconds.

Without the mastery (Going Fervor instead) you'd have 68.5% reduction.

A 3.25 second root would last about 1 second.

If you're playing another champion going for CoC you can have 55.375% reduction.

A 3.25 second root would last about 1.5 seconds.