Q. I haven’t been able to find anything in CMOS about how to punctuate a sentence that begins with a dependent clause that consists of two independent clauses introduced by a word like “If.” I’m a proofreader for court reporters. One of my clients consistently puts a comma within this type of dependent clause, but I don’t think the comma should be there. The comma I’m asking about is the comma after the word “valve” in the following example: “If you had to replace a leaking valve, and it had external insulation, how long would that work take?” To me, the part about external insulation is restrictive and should not be preceded by a comma. I’d like to find some authoritative confirmation of that. Thank you.
A. We agree that the first comma in your example is best omitted, not because the clause following and is restrictive (and doesn’t normally work that way) but because that comma competes with the one that is normally required after an if clause. So:
If you had to replace a leaking valve, how long would that work take? (The comma is required; see CMOS 6.26.)
If you had to replace a leaking valve and it had external insulation, how long would that work take? (A comma after “valve” would be strictly correct but is usually best omitted.)
To make a long story short, omitting the comma after valve from your example makes it clearer that the introductory If applies to both clauses (i.e., “you had . . .” and “it had . . .”). For a similar take, see this previous Q&A (which refers to the 17th ed. but still applies).
To understand how all this works, it might also help to consider your example sentence with that instead of and (though in your situation any changes to wording presumably wouldn’t be an option):
If you had to replace a leaking valve that had external insulation, how long would that work take?
In that case, that introduces a restrictive relative clause (see CMOS 6.29). And though it doesn’t change the meaning of your example, the word that can help ensure a restrictive reading of the “insulation” clause.
We’ll try to clarify these options in a future edition of CMOS.