The Z stack is the latest evolution on the T-stack kit from YoYoFactory. This is a stand alone stack, it does not need to be attached to a standard stack, it does it all by itself. It has a hole in the center to make center positioning your fingers easy and the outer lip has a indentation to keep your fingers from impacting the edge of the yo-yo slowing it down. An amazing upgrade. Fits full size YoYoFactory yo-yos that work with hubstacks. Comes with two caps and two o-rings.