Double Stack : Arguably the state of the art Rollerblock configuration at this time, especially when used with the state of the art Grade 3Superball upgrade. The Double Stack system uses an additional set of Series 2 bodies, inverted, placed on top of the ball bearing. Thus, a "sandwich" is formed with a block body on top and bottom, and a ball in between. This system can make startling improvements in component performance. The additional bodies are available by themselves at reduced price.