These handheld sharpening stones are an original design by Shosui Takeda's grandfather.  The handle was an addition to a standard stone design as it was meant for a handy way to hold the stones, but also to protect the wrist from the sharp blades. Takeda Hamono has tested out hundreds of stones with their knives, and found that these are optimal for their knives for easy use. #400 & #1200.