- This sort was originally used to sort computer punch-card decks.
- It is currently used for multi-key sorts (eg., year/month/day)
- Consider each digit of the number as a separate key

**Idea 1:** Sort on most significant digit n, then sort on
digit n-1, etc.

**Problem:** For old sorters, sort into 10 bins, but subsequent
recursive sorts require all 10 bins. Operator must store the other 9 piles.

**Idea 2:** Sort on least significant digit, then sort on
next least significant digit, etc.