Grouping of decimal numbers
Suppose we have a lot of 4-digit decimal numbers, 0000 to 9999
Want to make a group of 10^2 (100) numbers
Could use 00xx (0000 to 0099), or 31xx (3100 to 3199), or 99xx (9900 to 9999), etc
Should not use (0124 to 0223) or (3101 to 3200) etc, because they do not form groups in the same way