How to use punctuation to present supplementary information

Commas, dashes and round brackets (parentheses) each perform more than one function. When we use them correctly, we help readers quickly and easily understand our writing. However, people often use punctuation incorrectly or inconsistently, which can make their writing confusing.

This article explains how to use these three types of punctuation to present supplementary information.

What is supplementary information?

As you’ll see from the examples in this article, our sentences often contain supplementary words or phrases. These words or phrases are not essential to the structure of the sentence, but they may still be important.

The technical term for supplementary material is “parenthetical”, which explains where the word “parentheses” – or round brackets – comes from.

To test whether information is supplementary, remove it from your sentence and then check that the sentence is still grammatically correct, and the meaning hasn’t changed.

Here are two examples:

My mum, born in 1937, grew up during the second world war.

In the sentence above, the information between the commas is supplementary. Removing “born in 1937” does not change the meaning of the sentence.

My mum, because she experienced rationing during the second world war, loves oranges.

In this second sentence, the information between the commas is important; it explains why my mum loves oranges. However, when “because she experienced rationing during the second world war” is removed the sentence is still grammatically correct. The meaning of the sentence is also unchanged, although the reason my mum loves oranges is no longer apparent.

Which type of punctuation to use

We use three different types of punctuation show our readers which material is supplementary:

• Commas.
• Round brackets (the technical name for round brackets is “parentheses”, which distinguishes them from square brackets that have a different function).
• Dashes (en dashes or em dashes).

Commas, round brackets and dashes all perform the same role; they separate supplementary material from essential information. What distinguishes the three types of punctuation is the level of emphasis they put on the extra information.

Commas

Commas are the most common punctuation to use to present supplementary information. They provide readers with the extra information without interrupting the flow of the sentence.

Use commas unless your sentence already contains other commas, as adding more will confuse your readers.

Round brackets

If commas won’t work in your sentence, round brackets are the best way to show readers that information (such as dates, sources or inessential details) is useful but inessential.

Dashes

Dashes add more emphasis to the supplementary information than commas or round brackets do, because they interrupt the flow of a sentence. If you want readers to pay attention to the supplementary information, use dashes.

There are two types of dashes: en dashes and em dashes.

En dash

The en dash gets its name from the length of the letter N in a printing press. An en dash looks like this: –

To insert an en dash, use CTRL + the hyphen on the number keypad. Alternatively, use Insert | Symbol | More Symbols | Special Characters | En Dash.

Em dash

The em dash is longer than the en dash. Its length is the same as the letter M in a printing press. An em dash looks like this: —

To insert an em dash, use ALT + CTRL + the hyphen on the number keypad. Alternatively, use Insert | Symbol | More Symbols | Special Characters | Em Dash.

How to use commas

Use a comma before and after the supplementary information. Here’s an example:

We have requested, and are still waiting for, an answer to our questions.

Don’t use commas within commas. If the supplementary information contains commas, replace the commas with round brackets or dashes, or separate the sentence into two sentences. Here’s an example:

Waka Kotahi, a government agency that invests in roads and rail, and walking, cycling and public transport, has its headquarters in Wellington. Incorrect

Here are three ways to rewrite this sentence to make it easier to read.

Waka Kotahi (a government agency that invests in roads and rail, and walking, cycling and public transport) has its headquarters in Wellington.

Waka Kotahi − a government agency that invests in roads and rail, and walking, cycling and public transport − has its headquarters in Wellington.

Waka Kotahi has its headquarters in Wellington. The agency invests in roads and rail, and walking, cycling and public transport.

How to use round brackets

Use a round bracket before and after the supplementary information. Here are two examples:

Muhammed Ali (1942–2016) was possibly the greatest athlete of all time.

Three-point seatbelts are the safest option, although two-point (lap) belts are still legal.

We also use round brackets to present an abbreviation, if the abbreviation is used again in the document.

The mixed member proportional system (MMP) is popular.

Occasionally, you may encounter an organisation that is now commonly known by its abbreviated form as its full name has become outdated. If the organisation is very well known you could choose not to write the full name. Alternatively, you could use the following approach:

CCS (formerly the Crippled Children’s Society) gave us good advice.

Avoid using round brackets within round brackets. If you need to separate information within a round bracket, use commas or dashes.

How to use dashes

Use an en dash or an em dash before and after the supplementary information.

En dashes and em dashes perform the same function, but organisations usually specify which type to use in their style guide. The style guide will also specify whether to put a space before and after the dash (a “spaced” dash) or whether to have no space between the text and the dash (an “unspaced” dash).

This is an example of a sentence that uses spaced en dashes:

When you are driving at night watch for cyclists – they can be hard to see – and stop to rest if you feel sleepy.

This is an example of a sentence that uses unspaced em dashes:

Some popular sports—skiing and mountain biking especially—can be very dangerous.

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