Use roman numerals with proper names (eg, Henry Ford III). Note that no comma is used before the numeral. However, arabic numerals should be used as designators in all other cases (eg, round 2, Table 4, year 5; see also 10.4, Capitalization, Designators) unless roman numerals are part of formally established nomenclature (see 15.0, Nomenclature).
Step I diet
Schedule II drug
level I trauma center
Axis I diagnosis
But: type 2 diabetes mellitus, phase 3 study
Use roman numerals for cancer stages and arabic numerals for cancer grades (see also 15.2, Nomenclature, Cancer). In pedigree charts, use roman numerals to indicate generations and arabic numerals to indicate families or individual family members (see also Pedigrees in 4.2.2, Visual Presentation of Data, Figures, Diagrams). Roman numerals also may be used in outline format (see 4.1, Visual Presentation of Data, Tables).
In bibliographic material (eg, references or book reviews), do not use roman numerals to indicate volume number, even though roman numerals may have been used in the original. However, if roman numerals were used in the original title or in an outline, refer to the title or outline as it was published, with roman numerals. Retain lowercase roman numerals that refer to pages in a foreword, preface, or introduction. Roman numerals may also be used to number supplements to journals, so that roman numerals appear adjacent to page numbers in references to the work. In this case, the roman numerals should be retained.
For the use of roman numerals in biblical and classical references, follow the most recent edition of the Chicago Manual of Style (see also 3.0, References).
The following list indicates the roman equivalents for arabic numerals. In general, roman numerals to the right of the greatest numeral are added to that numeral, and numerals to the left are subtracted. A horizontal bar over a roman numeral multiplies its value by 1000.