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Commas, Nonessential Elements

You might be asking yourself why commas would delineate 鈥渘onessential elements鈥� at all: doesn鈥檛 concision call for removing whatever is unnecessary?

It does. But 鈥渘onessential鈥� in this case does not refer to information that is extraneous to your meaning; rather, it refers to supplementary (or 鈥渘onrestrictive鈥�) information about something鈥攁s opposed to information that specifies a subset or item, known as 鈥渆ssential鈥� or 鈥渞estrictive鈥� information. One of the comma鈥檚 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鈥搉onessential distinction is the choice between that and which.

Essential Tools: 鈥淭hat鈥� vs. 鈥淲hich鈥� and 鈥淲ho鈥� vs. 鈥�, Who鈥�

Simply put, that indicates essential (specifying) information, while 飞丑颈肠丑鈥�preceded by a comma鈥攊ndicates nonessential (supplementary) information:

1. 鈥淭he rebel groups that banded together鈥� = not all of the rebel groups banded together; right now, we鈥檙e only talking about those that did
2. 鈥淭he rebel groups, which banded together鈥� = all of the rebel groups in fact banded together; we鈥檙e talking about all of them

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

1. 鈥淭he rebels who banded together鈥� = not all of the rebels banded together; right now, we鈥檙e only talking about those who did
2. 鈥淭he rebels, who banded together鈥� = all of the rebels in fact banded together; we鈥檙e 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 鈥淭hat鈥� and 鈥淲hich鈥�

We frequently encounter constructions that do not actually contain 鈥渢hat鈥� or 鈥渨hich鈥� 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 鈥渨hich鈥�:          

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

Even without the 鈥渨hich,鈥� 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鈥攏ot because it鈥檚 superfluous to the discussion (it might or might not be) but because it鈥檚 not there to help us differentiate this treaty from any other; it鈥檚 just additional information about the only treaty this sentence could be talking about.

Version 2 contains an implicit 鈥渢hat鈥�:

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

With or without 鈥渢hat,鈥� 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): "," Purdue OWL
• Exercises: "," Simple Writing
• Handout (printable): "," University of North Carolina Chapel Hill