Base 61

In base 61, each digit position represents a power of 61: the rightmost digit counts individual ones, the next counts groups of 61, the next counts groups of 61×61, and so on — the same place-value idea as base 10, just with 61 distinct digits instead of ten.

Base 36

Base 36 works the same way: the rightmost digit counts ones, the next counts groups of 36, the next counts groups of 36×36, and so on, using 36 distinct digits.