Since Hex (base 16) uses 0-9A-F, and (I'm assuming here) Base 17 uses 0-9A-G and so on. What symbols are used once 0-9A-Z are all used up.
Well, look at base 64: 0-9, A-Z, a-z and then a few symbols depending on the context. (Base64 for the web tends to be different to other schemes to avoid URL/HTML encoding issues.)