Here is the rule again, in case you missed it: Essential elements in a sentence should not be enclosed in commas. Nonessential elements in a sentence should be enclosed in commas.

Last time, we applied the rule to clauses. Today we’ll look at essential and nonessential phrases (a phrase is two or more related words with no subject and verb).

Let’s start with this sentence: The guy seated next to me wouldn’t stop talking. There are no commas because seated next to me is an essential phrase. It identifies which “guy” we mean. Without it we’d have only The guy wouldn’t stop talking, which doesn’t tell us much.

But consider this: Ezra Blung, the guy seated next to me, wouldn’t stop talking. Because we now know the man’s name, the guy seated next to me becomes nonessential. As the commas signify, the phrase contains supplementary information, and the sentence would have the same meaning without it.

Commas are easy for some to overlook, but an omitted or out-of-place comma can change a sentence’s meaning. Here is an example: Complete the job, as directed. The comma after job tells us that the phrase as directed is nonessential. The sentence says that you have been directed to do a job, and implies that how you do it is up to you. But what if we took out the comma: Complete the job as directed. Now as directed is essential, and the sentence is saying something more severe: Do the work, and make sure you do it the way you were told to do it.

Remember that essential and nonessential are technical terms. Some authorities prefer restrictive and nonrestrictive, perhaps to avoid the sort of confusion that may result from analyzing a sentence like this: A comma, which never ends a sentence, signals a pause.

In that example, which never ends a sentence is nonessential, and the crux of the sentence is, A comma signals a pause. That is true, but a period also signals a pause. Perhaps the key difference between commas and periods is that a comma never ends a sentence.

So how could such an essential fact be termed “nonessential” in a sentence that describes a comma? It’s because we are using grammatical terminology: nonessential refers to sentence structure only.

Information essential to human understanding is often found in phrases and clauses that are technically nonessential, as seen in the comma sentence above. But that sentence would be improved by making its less important fact nonessential: A comma, which signals a pause, never ends a sentence.

Memo to fledgling writers: If you find that you’ve disclosed an essential fact in a technically nonessential phrase or clause, you may want to write a new sentence.