They are called cardinal prefixes. https://en.wikipedia.org/wiki/Numeral_prefix https://en.wikipedia.org/wiki/Numeral_prefix According to that page, both hexad

They are called cardinal prefixes. https://en.wikipedia.org/wiki/Numeral_prefix https://en.wikipedia.org/wiki/Numeral_prefix According to that page, both hexadeca- and decahexa- are valid. I think hexadeca- is more common. You gave on example, another is https://en.wikipedia.org/wiki/IUPAC_numerical_multiplier https://en.wikipedia.org/wiki/IUPAC_numerical_multiplier.