Commas, Nonessential Elements

You might be asking yourself why commas would delineate “nonessential elements” at all: doesn’t concision call for removing whatever is unnecessary?

It does. But “nonessential” in this case does not refer to information that is extraneous to your meaning; rather, it refers to supplementary (or “nonrestrictive”) information about something—as opposed to information that specifies a subset or item, known as “essential” or “restrictive” information. One of the comma’s jobs is to distinguish between these types of information.

The idea is harder to describe than to illustrate. Perhaps the most familiar manifestation of the essential–nonessential distinction is the choice between that and which.

Essential Tools: “That” vs. “Which” and “Who” vs. “, Who”

Simply put, that indicates essential (specifying) information, while which—preceded by a comma—indicates nonessential (supplementary) information:

1. “The rebel groups that banded together” = not all of the rebel groups banded together; right now, we’re only talking about those that did
2. “The rebel groups, which banded together” = all of the rebel groups in fact banded together; we’re talking about all of them

This same consideration determines whether or not to put a comma before who:

1. “The rebels who banded together” = not all of the rebels banded together; right now, we’re only talking about those who did
2. “The rebels, who banded together” = all of the rebels in fact banded together; we’re talking about all of them

Note that the essential vs. nonessential distinction does not play a role in selecting "who" vs. "whom" (both of which can introduce restrictive or nonrestrictive information).

Elided “That” and “Which”

We frequently encounter constructions that do not actually contain “that” or “which” but that are nevertheless able to maintain the distinction between essential and nonessential information by including or omitting commas. Take this example:

1. The peace treaty, signed by all the belligerents, lasted for more than 400 years.
2. The peace treaty signed by all the belligerents lasted for more than 400 years.

Version 1 contains an implicit “which”:          

The peace treaty, [which was] signed by all the belligerents, lasted for more than 400 years.

Even without the “which,” the commas indicate that this sentence is about one particular peace treaty that, it just so happens, was signed by all the belligerents. Placed between commas, the information about the signatories is nonessential or nonrestrictive—not because it’s superfluous to the discussion (it might or might not be) but because it’s not there to help us differentiate this treaty from any other; it’s just additional information about the only treaty this sentence could be talking about.

Version 2 contains an implicit “that”:

The peace treaty [that was] signed by all the belligerents lasted for more than 400 years.

With or without “that,” the absence of commas here indicates that this same information is now essential insofar as it specifies the treaty in question, distinguishing it from other treaties. That is, the information about the signatories restricts the statement about how long the treaty lasted to only that treaty signed by all the belligerents; this statement does not apply to any of the other treaties this sentence could otherwise have been talking about.

We might expect such a piece of information to be followed by a contrasting one:

The peace treaty signed by all the belligerents lasted for more than 400 years; those treaties signed by only some of the belligerents fell apart much more quickly.

Nonessential Elements Commas Links

• GWC writing mechanics module: Commas
• Handout (printable): "Commas with Nonessential Elements," Purdue OWL
• Exercises: "Comma Quiz with Non-essential Elements," Simple Writing
• Handout (printable): "Commas," University of North Carolina Chapel Hill