Base 21

In base 21, each digit position represents a power of 21: the rightmost digit counts individual ones, the next counts groups of 21, the next counts groups of 21×21, and so on — the same place-value idea as base 10, just with 21 distinct digits instead of ten.

Base 3

Base 3 works the same way: the rightmost digit counts ones, the next counts groups of 3, the next counts groups of 3×3, and so on, using 3 distinct digits.