Q. According to CMOS 6.51, “Expressions of the type that is are traditionally followed by a comma. They are best preceded by an em dash or a semicolon rather than a comma, or the entire phrase they introduce may be enclosed in parentheses or em dashes.” My question is this: Would it still be acceptable to use a comma in such expressions rather than the em dash or parentheses? Thank you!

A. Technically, yes: two commas would still be considered correct. But the problem with that first comma—and the reason we discourage it—is that unlike dashes, semicolons, and opening parentheses, which are forward looking, commas tend to be backward looking. For example,

The committee, that is, its more influential members, wanted to drop the matter.

Does the phrase “that is” in the example above belong with the words that come before it, or does it belong to the words that follow? A stronger mark solves this potential for a momentary misreading by providing more structure to the sentence:

The committee (that is, its more influential members) wanted to drop the matter.

Another solution is to simply omit the second comma:

The committee, that is its more influential members, wanted to drop the matter.

That last approach is fine for casual prose, but formal prose usually calls for the more structured punctuation choices recommended in CMOS.