If a heading is in roman numerals, is an in-text cross reference also in roman, or arabic, or spelled out

Question

Where in the Chicago Manual of Style is the answer to my subject question?

I looked in the CMOS, but could not find the answer to my question.

The book is nonfiction, self help divided into parts with chapters. In headings, the parts are numbered Part I, Part II, Part III, and so on.

If I cross reference within text, what is the CMOS recommendation?

• Recall in part I, ... (roman, same as in headings)
• Recall in part 1, ... (arabic)
• Recall in part one, ... (spelled out)

Note: I know that the overarching rule is consistency and that house style overrules whatever external authority is being used. When developing a house style, it's helpful to know if a style rule is the same as or an exception to CMOS (or other style authority).

Answer 1

You should certainly follow the same style used in the heading if there are multiple levels, and different systems are used for numbers. It avoids confusion.

If it's a less structured document, like the chapter titles of a book, you have more flexibility. I'd suggest either spelling it out or using the style from the chapter heading.

Chapter XX

In Chapter XVIII, we examined the methods for the investigation.

Or...

Chapter XX

In Chapter Eighteen, we examined the methods for the investigation.

You might see a mix of number styles in cases where the layout person decided the chapter titles would look nice in Roman numerals, but I wouldn't recommend writing that way.

It's probably easiest to stick to Arabic numbers, and then just follow the usual rules for writing or spelling numbers.

Answer 2

In CMOS 16th edition, section 9.29 (Numbered divisions in legal instruments) comes (as best I can determine) to answering my question:

"Arabic or roman numerals are sometimes used to distinguish divisions within legal instruments and other documents. ... A mixture of arabic and roman numerals sometimes distinguishes small from larger divisions."

The example they provide suggests choosing how you want to treat it:

According to the Constitution of the United States, article 2, section 4 (or Article II, Section 4)