Base 19

In base 19, each digit position represents a power of 19: the rightmost digit counts individual ones, the next counts groups of 19, the next counts groups of 19×19, and so on — the same place-value idea as base 10, just with 19 distinct digits instead of ten.

Base 51

Base 51 works the same way: the rightmost digit counts ones, the next counts groups of 51, the next counts groups of 51×51, and so on, using 51 distinct digits.