Commas have various uses. Generally, they break up the sentence into meaningful units. In speech, we use a pause to perform this function. It may be useful to think of a comma as the written equivalent of a pause in spoken language.
Some of the uses of commas are as follows:
To separate items in a list
Example: Vast sums were raised to support the relief and rehabilitation effort throughout Europe, North America and East Asia..
Commas are used between each item, but not usually before the final ‘and’ unless the list is complex in some way.
Example: From the beginning there was a certain bias in the sorts of interventions favoured by the relief agencies towards those which were highly visible, photogenic, and focused on the poor, women and children.
To separate extra information from the main idea of the sentence
Example: The figures, given in Table 3, clearly demonstrate that wheat exports have remained stable.
Or: The figures, which are given in Table 3, clearly demonstrate that wheat exports have remained stable.
The information (which has been underlined) between commas in the above sentences could be omitted without the main idea of the sentence being lost.
To separate introductory clauses which denote purpose
This is where the introductory clause is an incomplete sentence and so on its own it makes no sense (eg ‘in order to’).
Example: In order to examine the changes in ownership after the move to limited liability, data was collected from the Ulster Bank’s Annual Return of Shareholders.
Example: To substantiate his opinion, the author draws on an impressively wide range of quantitative as well as qualitative sources.
Note that these clauses of purpose (‘To do something’ and ‘In order to do something’) are additional to the main part of the sentence. If the sentence begins with this information, then the clause of purpose must be followed by a comma. However, if the clause follows the main part of the sentence (see the example below) you do not usually use a comma.
Example: The team examined the video footage, the teachers’ commentaries and the children’s writing in order to notice what emergent categories connecting drama and writing were apparent in the data.
These clauses of purpose are commonly used when writers are describing their research methods.
To separate linking words and phrases
Words that indicate the relationship between parts of the sentence, eg however, consequently, therefore, in addition, in contrast, on the other hand.
Example: However, any writer might be dissatisfied with their own work.
Example: Any writer, however, might be dissatisfied with their own work.
Example: Therefore, Plutarch’s spiritual world-view can function as a constructive beginner’s guide or supplementary aid to Christian theology.
Example: To be free, therefore, is to embrace moral duty.
Example: In contrast, all states further down the list, with the exception of California, allow aliens resident in the state the same property rights as citizens.
To introduce a clause that begins with an –ing form verb
Example: The margin of income over expenditure has reduced further, resulting in a small surplus on continuing operations, of £2.5m.
Example: Since 1998, there have been numerous incidents of policy and administrative failure, leading to widespread public discontent with the government.
‘ing’ clauses can be used to add more information to the main sentence. In research writing, they are normally placed at the end of the sentence. In literary narratives they are often found at the beginning of the sentence for dramatic effect!
To separate two clauses where one is dependent on the other for meaning
By itself the dependent clause makes no sense. We always do this when words such as, ‘when’, ‘while’, ‘whereas’, ‘although’, ‘because’ and ‘if’ come at the beginning of the sentence.
Example: Although China began the transition from coal to oil in the late 1950s, it will remain heavily dependent on coal for the foreseeable future.
Example: If a foreign investor believes that a host government has breached any of these obligations, the investor can seek remedy through the courts of the host country.
In the examples above, a comma is necessary because the introductory part of the sentence has no meaning on its own.
Also, if you put the dependent part of the sentence second, a comma is normally used to separate the two parts of the sentence (except for with ‘because’ and ‘if’):
Example: In development, expert knowledge and contextual knowledge are seen as important for success, although the extent to which they are valued equally is an open question.
In this example a comma is necessary.
Example: There is little point in setting targets if we do not know through what actions the outcomes can be influenced.
Here no comma is used.
When not to use commas
In a defining clause (where the information is necessary, not additional).
Example: European policymakers had mapped out a set of arguments which would inform their subsequent critique and negotiating stance.
Example: The scheme is aimed at PhD students who have a proven track record in the field of economics.
These sentences are examples of defining clauses. All the information is necessary to convey the full meaning and without ‘which’ or ‘who’ the sentences would not make sense. Thus, no comma is used before ‘which’ or ‘who’.
Example: The scheme is aimed at PhD students, who have a proven track record in the field of economics.
The use of a comma here suggests that the information after the comma is extra information, ie it is not necessary. This would suggest that all PhD students have a proven track record in the field of economics. This is clearly untrue (unless of course this information is published somewhere in the School of Economics).
For further information on punctuation and online practice exercises you should look at the guide below which is sponsored by the Capital Community College Foundation:
NB. You should also find out if there are any specific punctuation conventions within your own discipline/department.