Talk:Cooldown reduction/@comment-29964258-20171218133734


 *  Ruby Sightstone ,  Cosmic Insight , and  Ingenious Hunter   stack additively with each other while  Perfect Timing   stacks multiplicatively with them.

I think I have discovered the reason for this. As per Patch notes for ruby and the first two runes, they are described as giving active item CDR, whereas Perfect Timing is simply described as reducing cooldowns.

So, Perfect Timing has a different method of reducing CDR, and perhaps is most synonymous with base cooldown reduction, such as hecarim's q's base CD going down by one after he hits someone with it, however, the other three give Active item CDR  (which is synonymous to real CDR), which applies seperately from, and thus multiplicitvely to, perfect timing.