Style Guide > 3 Punctuation > [3.4] Comma ,
[3.4] Comma ,
3.4 When to use a comma
The purpose of the comma is to assist the reader to properly interpret the text. Its primary function is to separate independent words or phrases. In general, if you can remove the word or phrase without changing the sense of what is left behind, then the comma is needed.
The main uses of the comma are to separate:
3.5 Punctuating a series
In a series consisting of three or more items, separate the items with commas. When a conjunction joins the last two items in a series, use a comma before the conjunction—it’s called a serial comma. Although, until quite recently, this extra comma was omitted, it’s use is now strongly encouraged. It is particularly important to use the serial comma when separating phrases, especially phrases that contain conjunctions, such as “and” or “or” (note the two ands in the third example below).
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Chapter 15 is an alphabetical reference to resources, procedures, and related topics.
You will need a 2 lb hammer, an M6 nut, and a little grease.
We have already made provisions for annual leave, occupational health and safety, and the superannuation guarantee.
3.6 Punctuating independent adjectives
Adjectives that are applied with equal importance to the noun are separated by commas. For example, “a long, steel rod” is both a “long rod” and a “steel rod”. In this example, the adjectives long and steel are applied independently on the noun rod. When adjectives are not independent, they are usually connected with a hyphen and applied as a unit on the noun, as in “a long-standing arrangement”.
3.7 Separate independent clauses
When two clauses that form complete sentences are connected by a conjunction, a comma is placed before the conjunction. For example, “Stocks of this reagent are low, but a new order is already in the system.” The two parts of the sentence each form complete sentences on their own—that is, they are independent clauses.
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Close the lid, and then insert the tab in slot A.
The feed is crushed. It is then transported by conveyor to the mill.
The feed is crushed and transported by conveyor to the mill.
The crushed feed is transported by conveyor from the crusher to the mill.
The test results are consistent with the findings of the study, but they are considered unreliable.
The camp is situated south of the township, and the access road runs west from the highway.
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Close the lid, then insert the tab in slot A. [then is not a conjunction]
The feed is crushed, and then transported by conveyor to the ball mill.
The camp is situated south of the township, accessed from the highway.
3.8 Separate non-restrictive phrases
The application of commas to phrases (fragments or incomplete sentences) is determined by whether the phrase is restrictive phrase or non-restrictive. An example of a non-restrictive phrase, which must be separated from the main clause with a comma, is: “The report is overdue, which is unusual.” The second phrase adds extra information, but it does change the meaning of the main clause.
Conversely, when the phrase “that confront the industry” is removed from the sentence “The report describes the issues that confront the industry.”, the meaning of the sentence changes to “The report describes (all or any) issues.” The phrase is restrictive, and so it must not be separated with a comma.
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The results, which are summarized in the following table, clearly demonstrate this theory.
3.9 Separate transitional words and phrases
Separate transitional words and phrases from the phrase with commas. Some common transitional words and phrases include however, such as, for example, and nevertheless.
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For example, use a comma in this sentence.
The road, however, is impassible.
The floor and roof RLs are at 100 m and 93 m, respectively.
Otherwise, the project will close.
3.10 End introductory phrases with a comma
Unless the phrase is short, separate introductory phrases with a comma. For example, when written in standard word order, the sentence “Don’t forget to remove the pin after you close the valve.” does not take the comma—the second phrase, starting with the word “after”, is restrictive. However, when the phrase is moved to the start of the sentence, it does: “After you close the valve, don’t forget to remove the pin.”
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In most large cities, you will find many fine restaurants.
In 2007, the company posted record earnings.
In 2007 the company posted record earnings.
3.11 Format numbers with commas
Use commas to format numbers of four or more digits. However, with numbers with more than three decimal places, and coordinates, use non-breaking spaces instead of commas.
3.12 At discretion of the writer
Commas can be introduced, at the discretion of the writer, to indicate a slight pause, if necessary, to assist with comprehension.
Also see When not to use commas.
[3.13] When not to use commas
3.13 Don’t separate a restrictive phrase or dependent clause
Do not separate a restrictive phrase phrase or dependent clause from the main clause with a comma.
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The processing plant uses technology that was developed by QIT in Canada. (not just any technology)
The project was ineffective because of inadequate resources. (the reason is important and related)
The study showed that the plan is feasible. (nonsensical if anything is removed)
The study will be concluded when the
contaminant tests have been completed. (it is concluded by something specific)
3.14 Don’t separate a verb from its subject
Do not use a comma between the verbs in a compound predicate.
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The conveyor takes the crushed feed from the crusher and delivers it to the mill.
The company supports many community programmes and enjoys an excellent relationship with the traditional owners.
The scanner checks your pass, and then it sends your information to the control centre.
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The conveyor takes the crushed ore from the crusher, and delivers it to the mill.
The company supports many community programmes, and enjoys an excellent relationship with the traditional owners.
The scanner checks your pass, and then sends your information to the control centre.
3.15 Don’t use commas in dates
Do not use commas in date formats.
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The project commenced in May, 1990.
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