A comma marks a slight break between different parts of a sentence. Used properly, commas make the meaning of sentences clear by grouping and separating words, phrases, and clauses. However, many people are uncertain about the use of commas and often sprinkle them throughout their writing without knowing the basic rules.
Here are the main cases when you should use a comma:
You should put a comma between the different items in a list, as in the following sentences:
Saturday morning started with a hearty breakfast of scrambled eggs, bacon, sausage, and French toast.
The school has a vegetable garden in which the children grow cabbages, onions, potatoes, and carrots.
The final comma in these lists (before the word ‘and’) is known as the ‘serial comma.’ Because of its long-standing traditional use by the university presses of Oxford and Harvard, it's also known as the 'Oxford comma' or the 'Harvard comma.' Not all writers or publishers use it - in fact, newspapers customarily do not - but many will argue that using it can make your meaning clearer. Take a look at this sentence:
The art department offers beginner classes in sculpting, watercolor, macramé and jewelry and metalcraft.
It isn’t clear from this sentence whether there is a class in 'macramé and jewelry' , a class in 'jewelry and metalcraft,' or separate classes in all three. This could be confusing for a prospective student. Adding the serial comma makes the meaning clear:
The art department offers beginner classes in sculpting, watercolor, macramé and jewelry, and metalcraft.
When a writer quotes a speaker’s words exactly as they were spoken, this is known as direct speech. If the piece of direct speech comes after the information about who is speaking, you need to use a comma to introduce the direct speech. The comma comes before the first pair of quotation marks. Note that the closing quotation marks follow the period:
Steve replied, "No problem."
You also need to use a comma at the end of a piece of direct speech, if the speech comes before the information about who is speaking. In this case, the comma goes inside the closing quotation marks:
"I don’t agree," I replied.
"Here we are," they said.
There are two exceptions to this rule. If a piece of direct speech takes the form of a question or an exclamation, you should end it with a question mark or an exclamation point, rather than a comma:
"Stop him!" she shouted.
"Did you see that?" he asked.
Direct speech is often broken up by the information about who is speaking. In these cases, you need a comma to end the first piece of speech (inside the quotation marks) and another comma before the second piece (before the quotation marks):
"Yes," he said, "and I always keep my promises."
"Thinking back," she added, "I didn’t expect to win."
See more about Punctuation in direct speech.
Commas are used to separate clauses in a complex sentence (a sentence that is made up of a main clause and one or more subordinate clauses).
The following examples show the use of commas in two complex sentences:
Having had lunch,
we went back to work.
I first saw her in Paris,
where I lived in the early nineties.
If the commas were removed, these sentences wouldn’t be as clear but the meaning would still be the same. There are different types of subordinate clause, however, and in some types the use of commas can be very important.
A subordinate clause beginning with ‘who,’ ‘which,’ ‘that,’ ‘whom,’ or ‘where’ is known as a relative clause. Take a look at this example:
who have young children
may board the aircraft first.
This type of relative clause is known as a ‘restrictive relative clause.’ Basically, a restrictive relative clause contains information that’s essential to the meaning of a sentence. If we removed the relative clause from the previous sentence, the whole point would be lost and we'd be left with the rather puzzling statement:
Passengers may board the aircraft first.
You should not put commas around a restrictive relative clause.
The other type of subordinate clause beginning with ‘who,’ ‘which,’ ‘whom,’ etc., is known as a ‘nonrestrictive relative clause.’ A nonrestrictive relative clause contains information that is not essential to the overall meaning of a sentence. Take a look at the following example:
who has two young children,
has a part-time job in the library.
If you remove this clause, the meaning of the sentence isn’t affected and it still makes perfect sense. All that’s happened is that we’ve lost a bit of extra information about Mary:
Mary has a part-time job in the library.
You need to put a comma both before and after a nonrestrictive relative clause.
Commas are used to separate a part of a sentence that is an optional ‘aside’ and not part of the main statement. For example:
Gunpowder is not, of course, a chemical compound.
Our favorite Gary Cooper movie, Sergeant York, is on TV at nine o'clock.
In these sentences, the role of the commas is similar to their function in nonrestrictive relative clauses: they mark off information that isn’t essential to the overall meaning. Using commas in this way can really help to clarify the meaning of a sentence. Take a look at this example:
Cynthia’s daughter, Sarah, is a midwife.
The writer’s use of commas tells us that Cynthia has only one daughter. If you removed Sarah’s name from the sentence, there would still be no doubt as to who was the midwife:
Cynthia’s daughter is a midwife.
If you rewrite the original sentence without commas its meaning changes:
Cynthia’s daughter Sarah is a midwife.
The lack of commas tells us that the name ‘Sarah’ is crucial to the understanding of the sentence. It shows that Cynthia has more than one daughter, and so the name of the one who is a midwife needs to be specified for the meaning to be clear.
If you aren’t sure whether you’ve used a pair of commas correctly, try replacing them with parentheses or removing the information enclosed by the commas altogether, and then see if the sentence is still understandable, or if it still conveys the meaning you intended.
You should use a comma after 'however' when however means 'by contrast' or 'on the other hand':
However, a good deal of discretion is left in the hands of area managers.
Don't use a comma after however when it means 'in whatever way':
However you look at it, existing investors are likely to lose out.