There are many occasions when a writer should use a comma: in lists or when describing a series of actions; when quoting direct speech; to separate clauses within a sentence; to mark out parenthetical phrases; as a vocative comma; or used in conjunction with however.
However, commas are often used too liberally or incorrectly or both, which can lead to sentences not having the meaning that the writer intended. In addition, some rules are harder and faster than others, which can lead to some confusion; although, there is always an option that is definitely correct if you are unsure.
Use a comma between items on a list. If it’s a short list of simple items, then you do not necessarily need to use a comma between your last item and the conjunction (usually and or or) in front of it. This is called the Oxford comma (as it used by the Oxford University Press), the Harvard comma, or just the plain old serial comma.
However, many writing styles (especially in the US) will expect that you do use the serial comma, for extra clarity. It is certainly a good idea to use the comma if the list involves complicated items.
Examples of listing comma
You can eat either the salmon, chicken or beef.
You can eat either the salmon, chicken, or beef. (Oxford comma here.)
Each of the above are correct. If you’re not sure, use the comma as it is never wrong.
However, for a slightly more complicated list of items, the serial comma is advisable:
Dave will eat the salmon, Craig will eat the chicken, and Arthur will have the beef.
Commas and quotation marks
This is another area in which there are stylistic differences, again dividing very roughly on either side of the Atlantic.
As a rule, a comma is placed between the reporting clause (which describes who spoke) and the reported clause (exactly what they said); for example, He said, “Get out!”
If the reported clause comes first in the sentence, the comma comes after. Whether it appears within or outside of the quotation marks is a stylistic choice; generally, writers in British English will place the comma outside, and American English writers, inside.
However, a comma shouldn’t be used if the quoted speech already ends with a question mark or exclamation mark, and that punctuation will always appear within the quotation marks; for example, “Great!” she replied.
Commas separating clauses
And the grammar-based migraines begin!
The rule of thumb is that the comma will mark off the information that is not necessary for the text to form a complete sentence; for example:
This sentence, which has taken me hours to write, forms part of the section on commas and clauses.
As you could write this as This sentence forms part of … and still make sense, commas are used to mark off the dependent clause (which has taken me hours to write).
However, if you choose to write this as two separate independent clauses, connected by a conjunction like and or but, you would also use a comma to divide them.
This sentence forms part of the section on commas and clauses, and it has taken me hours to write.
Comma and because
As conjunctions like because often get involved in these sentences, writers are sometimes unsure about whether a comma should be used. As because will usually describe the reason for the first part of the sentence, no comma should be used.
They stayed indoors because it was raining outside.
Simple as. However, things become a little more complicated if the first part of the sentence is negative.
They didn’t go outside because it was raining means that although they actually went outside, the rain was not the reason, whereas inserting a comma will mean that they didn’t go outside at all, i.e. They didn’t go outside, because it was raining.
Sneaky, isn’t it?
Comma and however
However is a word that often trips writers up as it can serve very different roles.
Cheese is a difficult substance to work with however you slice it
has a very different meaning to
Cheese is a difficult substance to work with; however, you slice it.
And the meaning changes again with another switch in punctuation:
Cheese is a difficult substance to work with, however: you slice it.
In the first sentence however without any surrounding punctuation simply means “no matter which way”.
In the second, however means “despite this” and therefore needs to come after a semi-colon as it starts a new clause, and is followed by a comma.
In the third example, however has a meaning closer to “on the other hand” and appears marked off by commas; with this type of usage, it could appear at the start of the sentence too, e.g. However, cheese is a difficult substance…
Comma and such as
The trick with using commas and such as again involves brushing up a little on grammar and the concept of restrictive and non-restrictive uses.
If the use of such as helps to narrow down the meaning of the word, i.e. a restrictive usage, then no comma should be used. This is because the such as should not be separated by a comma from whatever it is being used to restrict.
However, if the such as is just being used to offer examples of the meaning, i.e. non-restrictive, then the comma goes in.
Examples of such as and commas
A beautiful face such as this could inspire poetry. (Restrictive: only a face like this could inspire poetry, not all faces.)
A face, such as the beautiful face of my wife, can tell you a lot about someone’s life. (Non-restrictive: all faces can tell a story – my wife’s is an example.)
Comma and so
A conjunction like so (as mentioned above) will often be used to join independent clauses together in one sentence. In this situation, so will also require a comma in front of it.
However, if the so is in fact the beginning of the subordinate conjunction so that, i.e. it connects a dependent clause to an independent one, then no comma is used. The really difficult part here is that the that is sometimes left out, which makes it easier to slip up.
Examples of commas and so/so that
He always spent a long time on his hair so (that) he felt attractive when he went out. (Subordinate conjunction)
He had been in the hairdressers for much longer than planned, so he had to call his girlfriend to say he would be ten minutes late. (Coordinating conjunction)
Comma and the date
This is another area in which transatlantic differences affect whether or not commas are used.
The American English convention is for the date to be written in order of month, date, then year; for example: October 21, 1972. In the UK, however, the date is written with the number first, i.e. 21 October 1972.
You’ll notice that there is no comma between date and year in the British version. However, in either version, there is no comma between month and year if no date is included; for example, October 1972.