Commas
The rules for the use of commas can sometimes be tricky. It is often about knowing where the main clause is, and what counts as subordinate or additional information. Here we explain how you can make your sentences more reader friendly by using commas wisely. The examples in this section are organised under two headings: when to use commas, and when not to use commas.
When to use commas
“Commas are the equivalent of changing gear when driving; you come to a point where you need to slow down a little or turn a corner. The commas help you negotiate these changes, but also, and perhaps more importantly, they enable you to take your reader along with you.”
(Peck and Coyle 2012: 54)
Separating the main clause from additional words and phrases
Sentences can be easier to follow for the reader if you use a comma to separate words and phrases coming before (or sometimes after) the main clause, especially if this phrase is long, as in example (1):
Less reader friendly
(1b) After careful study of the data we conclude that the correlations do not tally.
More reader friendly
(1b) After careful study of the data, we conclude that the correlations do not tally.
The comma after data helps the reader to locate the subject of the sentence.
Likewise, it is often a good idea to use commas if a word or phrase ‘interrupts’ a clause or structure, as in (2):
Less reader friendly
(2a) We are forced to conclude after careful study of the data that the correlations do not tally.
More reader friendly
(2b) We are forced to conclude, after careful study of the data, that the correlations do not tally.
In this way, the reader is able to extract the main point easily.
This principle also applies to phrases used in ‘apposition’, i.e. consecutive phrases, usually noun phrases, the second one defining or modifying the first. The apposition is in bold in (3b):
Less reader friendly
(3a) The constant Ke is determined by the magnetic and electric operating conditions.
More reader friendly
(3b) The constant, Ke, is determined by the magnetic and electric operating conditions.
Separating the main clause from subordinate clauses
It is common to separate the main clause and subordinate clause with a comma when the subordinate clause (in bold) comes first, as in (4a–b):
Less reader friendly
(4a) Since most pollutants like to attach to fine sediment they can be transported over long distances.
More reader friendly
(4b) Since most pollutants like to attach to fine sediment, they can be transported over long distances.
However, if the main clause precedes the subordinate clause, it is not necessary to use a comma, although people often do, and both types are considered acceptable if they make sense:
(4c) Most pollutants can be transported over long distances since they like to attach to fine sediment.
(4d) Most pollutants can be transported over long distances, since they like to attach to fine sediment.
In general, avoid unnecessary commas as they can make the page too ‘busy’ and difficult to read.
Finally, use commas to separate non-restrictive relative clauses from the main clause, as in (5):
Incorrect
(5a) Granule cells transmit information to Purkinje cells which in turn provide the output of the cerebellum to deep cerebellar nuclei.
Correct
(5b) Granule cells transmit information to Purkinje cells, which in turn provide the output of the cerebellum to deep cerebellar nuclei.
Read more about commas with relative clauses here .
What is the Oxford comma?
The Oxford comma is a comma used before words such as and, but,and or to separate clauses or items in a list. It is so named because it is associated with Oxford University Press, which advocates its use. It is sometimes called a ‘serial comma’ because it separates a series of elements in a sentence. The Oxford comma is optional, and some people prefer not to use it, but it can be very useful, especially in lists, for example (6):
Less reader friendly
(6a) We demonstrated the utility of our approach by printing micro-supercapacitors with large surface area, fast ion transport and excellent cycling and temperature stability.
More reader friendly
(6b) We demonstrated the utility of our approach by printing micro-supercapacitors with large surface area, fast ion transport, and excellent cycling and temperature stability.
The Oxford comma is useful here because it makes it clear that there are three items in this list, and that the word stability is connected to both cycling and temperature:
(6b) We demonstrated the utility of our approach by printing micro-supercapacitors with [large surface area], [fast ion transport], and [excellent cycling and temperature stability].
When not to use commas
Read here about other common problems in sentence structure.
There are three contexts where you need to make sure not to use a comma:
In restrictive relative clauses
Incorrect:
(7a) This is the model, that/which is used to calculate the mass torque.
Correct:
(7b) This is the model that/which is used to calculate the mass torque.
Read more about commas with relative clauses here.
Before that-clauses
Incorrect:
(8a) It is thought, that these climatic events will continue.
Correct:
(8b) It is thought that these climatic events will continue.
Between a subject and verb
No matter how long the subject is, it should not be separated from the verb with a single comma. The long subject is within square brackets in examples (9a)–(9c):
Incorrect:
(9a) [The most significant discoveries made in this period], were in the field of space exploration.
Correct:
(9b) [The most significant discoveries made in this period] were in the field of space exploration.
However, if the subject ends with a restrictive relative clause or other parenthetical information, there will be a pair of commas between the subject and the verb, as in (9c). This follows the same principle as in examples (2)–(3) above.
Correct:
(9c) [The most significant discoveries made in this period, further described in Section 2], were in the field of space exploration.
References
Peck, J. and M. Coyle (2012) Write It right: The secrets of effective writing. (2nd Edition). Palgrave Macmillan.