When to use a comma: non-restrictive phrases
This is the first in a series of articles about best-practice use of commas. The comma is by far the most important of all the marks of punctuation because, when used properly, it reduces the risk of ambiguity and misreading.
The main purpose of the comma is to show grammatical independence of words and phrases. So what is “grammatical independence”, I hear you ask? You test for independence by removing the word or phrase from the sentence. If the meaning of what remains is unchanged, then the bit that you removed is independent of the main sentence and must be separated with a comma or another punctuation mark that signifies independence (e.g. semicolon or full stop). If a phrase, it is said to be “non-restrictive”. The best way to explain it is with an example.
Consider the sentence “Offers, which can be submitted online, must be received before 11 AM.” Now, if you remove the phrase between the commas, it reads “Offers must be received before 11 AM.”, which is entirely true and unaffected by whether or not offers are submitted online. So the phrase “which can be submitted online” is non-restrictive and must be separated from the main clause with commas.
Okay, so why is this so important? Well, if we aren’t clear about whether one part of a sentence restricts the meaning of the rest of the sentence, our meaning can become muddled. Consider the following sentences:
The projects, located in South Gippsland and the Wimmera, are part-funded by the state government.
The commas indicate that the location of the projects is incidental to the main statement that they are government-funded. The phrase is non-restrictive. Whereas, if you remove the commas, the sentence reads:
The projects located in South Gippsland and the Wimmera are part-funded by the state government.
Whether or not this was intended, the location now appears to qualify which projects are government-funded—that is, only the projects in South Gippsland and the Wimmera are supported. The phrase is now restrictive. The two sentences have very different meanings, and the second will likely leave the reader confused about whether this was intended.
In addition to the commas, as an aid to interpretation, it is also good practice to follow the grammatical convention of using “which” to introduce a non-restrictive phrase and “that” to introduce a restrictive phrase. So with a few minor tweaks, we can make our meaning much clearer:
The projects, which are located in South Gippsland and the Wimmera, are part-funded by the state government.
[non-restrictive phrase, separated by commas, introduced with “which”]
or
Only projects that are located in South Gippsland and the Wimmera are part-funded by the state government.
[restrictive phrase, no commas, introduced with “that”]
Other uses of the comma (in addition to separating a non-restrictive phrase from the main clause) include:
Separating independent clauses
Separating items in a series
Separating independent adjectives
Separating a transitional phrase or word from the main clause
Ending an introductory phrase
Ending a conditional clause
There are examples of most of these in this article. See if you can find them. We’ll explore these other uses of the comma later in this series. You’ll find plain language explanations of the jargon in the Doxical Style Guide for Miners and Geoscientists.
