Base 18

In base 18, each digit position represents a power of 18: the rightmost digit counts individual ones, the next counts groups of 18, the next counts groups of 18×18, and so on — the same place-value idea as base 10, just with 18 distinct digits instead of ten.

Base 28

Base 28 works the same way: the rightmost digit counts ones, the next counts groups of 28, the next counts groups of 28×28, and so on, using 28 distinct digits.