Q: When a quote comes right after a verb like “said” or “asked,” we use a comma (e.g., God said, “Thou shalt not kill”). But do we still need a comma if we don’t use a verb (e.g., God’s statement “Thou shalt not kill” is one of the Ten Commandments)?
A: No, you wouldn’t use a comma to introduce the quotation in your second example.
The Chicago Manual of Style (16th ed.) notes (as you point out) that a quotation “in the form of dialogue or from text is traditionally introduced with a comma.”
Elsewhere, the manual says, “a comma is used after said, replied, asked, and similar verbs.”
But not every quotation requires an introductory comma.
For instance, the Chicago Manual says no comma is needed before a quote introduced by “that,” “if,” “whether,” or a similar conjunction.
We’ll invent some examples: “He wondered if ‘Thou shalt not kill’ was the fifth or sixth commandment” … “She asked whether ‘Thou shalt not kill’ or ‘You shall not murder’ was the proper wording” … “How can a murderer believe that ‘Thou shalt not kill’ is God’s law?”
And your sentence—“God’s statement ‘Thou shalt not kill’ is one of the Ten Commandments”—illustrates another kind of quotation that doesn’t need a comma.
In this case the quotation (“Thou shalt not kill”) is the explanatory equivalent of the subject (“God’s statement”).
An English teacher would call the quotation an appositive—something placed in apposition to a noun or noun phrase. Grammatically, “apposite” means equivalent (not to be confused with “opposite”).
Sometimes these explanatory equivalents are surrounded by commas and sometimes they’re not.
The Chicago Manual explains the situation in a nutshell here (brace yourself for more grammatical terminology):
“A word, abbreviation, phrase, or clause that is in apposition to a noun (i.e., provides an explanatory equivalent) is normally set off by commas if it is nonrestrictive—that is, if it can be omitted without obscuring the identity of the noun to which it refers. …
“If, however, the word or phrase is restrictive—that is, provides essential information about the noun (or nouns) to which it refers—no commas should appear.”
In plain English, put commas around an explanatory equivalent that’s dispensable—one that could be dropped without losing the point of the sentence. But don’t put commas around one that’s essential to the point.
In your sentence (“God’s statement ‘Thou shalt not kill’ is one of the Ten Commandments”), the appositive (“Thou shalt not kill”) is essential, so you don’t need commas. If you dropped the appositive, the point of the sentence would be lost.
In a section on “maxims, proverbs, mottoes and other familiar expressions,” the Chicago Manual gives examples of two appositive sayings, one with commas and one without:
Commas used: “Tom’s favorite proverb, ‘A rolling stone gathers no moss,’ proved wrong.”
Commas omitted: “The motto ‘All for one and one for all’ appears over the door.”
“My husband, John, will be joining us for dinner.” (Commas used; you have only one husband, so the name isn’t essential information.)
“My friend Susan will be joining us for dinner.” (Commas omitted; she’s not your only friend, so her name is essential to identify which friend.)
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