What base is Roman Numerals?

Question

What is the base for Roman Numerals? It starts off with unary then goes back and forth between multiples of 5 and 10.

Answer 1

Since it is not a positional system, it does not really make sense to speak of a unique "base" for Roman numerals, mathematically. One could describe them as "mixed base 5-10" (most commonly) or even "mixed base 2-5", but neither of those are formal descriptions.

Answer 2

I disagree with Henning's and J.M.'s identification of positional systems and systems with a base. There are examples of non-positional systems with a single base (10 in both cases): Egyptian numerals and Chinese numerals. The first footnote in the Wikipedia article on Roman numerals calls them "a decimal system in which the number 5 is an auxiliary base".