Fantastic and unique puzzle based on a 1911 design. The object is to get each of the 16 columns of numbers to add to 40 at the same time. Can you find the combination. There is a base ring and then 4 rings that rotate on top. To complicate matters, the top sets of rings are all notched, so they will expose or hide some of the numbers on the ring below as the rings are turned. Makes for a lot of possible combinations! It essentially turns the 16 columns into 24 possible columns. Its quite a challenge. Beautifully made in USA at our Spring Hill shop with highest quality alder rings and laser engraved numbers. Ergonomically sized to fit the palm nicely. The diameter is 11cm . Each ring is cm " thick alder. To the best of my knowledge there is only 1 possible solution.