Grouping of binary numbers
Suppose we have a lot of 4-bit binary numbers, 0000 to 1111
Want to make a group of 2^2 (4) numbers
Could use 00xx (0000 to 0011), or 01xx (0100 to 0111), or 10xx (1000 to 1011), or 11xx (1100 to 1111)
Should not use (0101 to 1000) or (1001 to 1100) etc, because they do not form groups in the same way