Base 10

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

Base 15

Base 15 works the same way: the rightmost digit counts ones, the next counts groups of 15, the next counts groups of 15×15, and so on, using 15 distinct digits.