PingMag did an interview with the creators of the Tenori-on that was overlooked when published, but there are a few interesting tidbits from the interview, like the fact that neither of the creators are musicians.
Yu said it took three years to develop the Tenori-on, and another three to get Yamaha on board with the production process. When throwing around design ideas, shapes like triangles were considered, but appeared too difficult to make usable, so they stuck with the square shape. Other interesting design aspects include making the Tenori wide enough so that the thumbs of the average adult hand meet in the middle of the Tenori when grasped, and that it was never intended to be a user-friendly music machine; it kind of just happened. Read the full interview over at [PingMag].