Base 62

In base 62, each digit position represents a power of 62: the rightmost digit counts individual ones, the next counts groups of 62, the next counts groups of 62×62, and so on — the same place-value idea as base 10, just with 62 distinct digits instead of ten.

Base 25

Base 25 works the same way: the rightmost digit counts ones, the next counts groups of 25, the next counts groups of 25×25, and so on, using 25 distinct digits.