Final answer:
The Stock system uses roman numerals to indicate the positive charge on the cation of ionic compounds with transition metals that have multiple oxidation states.
Step-by-step explanation:
The Stock system, used to name ionic compounds with transition metals that have multiple oxidation states, uses roman numerals to indicate the amount of positive charge on the cation. This system is only used for elements that form more than one common positive ion. For example, Fe²+ is called the iron(II) ion, while Fe³+ is called the iron(III) ion.