True 3 dimensional problem. Each 4X4 square is one plane of a 4X4X4 cube. Consider the squares stacked on top of each other. There are 12 planes in this problem each of which is a 4X4 square which must have no repeating characters. Some of the planes are highlighted. There are 16 possible characters so I use a standard hexadecimal character set: 0,1,2,…,9,a,…,f.