The comma is used very frequently but is also often used incorrectly.
There are four distinct uses of the comma:
A listing comma is used as a kind of substitute for the word ‘and’ or sometimes for the word ‘or’ in a list when three or more words, phrases or even complete sentences are joined by the word ‘and’ or ‘or’.[The colours in the Union Jack flag are red, white and blue.]
A joining comma is only slightly different from a listing comma and is used to join two complete sentences into a single sentence, when it must be used by one of the connecting words ‘and’, ‘or’, ‘nor’, ‘but’, ‘while’, ‘so’ and ‘yet’. [I could tell you the truth, but I will not.]
The gapping comma is used to show that one or more words have been left out when the missing words would simply repeat the words already used in the same sentence. [Some English writers use punctuation correctly; others, not.]
The bracketing comma always comes as a pair and is used to mark off a weak interruption of a sentence – that is, an interruption which does not disturb the smooth flow of the sentence and could be removed and still leave the sentence complete and making good sense. [This web site, I would suggest, contains much useful information and advice.]
Other uses of comma are:
Use commas to separate items in a series.
Example: Our itinerary included Rome, London, and Madrid.
Use a comma before and, but, or, nor, for, so, and yet, when they join independent clauses (unless the clauses are short).
Example: The story gets off to a slow start, but it gets exciting toward the end.
Use commas to set off nonessential clauses and phrases.
Example: My father, who started this company, really knows his stuff.
Use a comma after introductory elements.
Examples: Well, how do you do? Before you leave, turn off the lights.
Use commas to set off an expression that interrupts a sentence.
Examples: The article in The Herald, our local paper, is about writing skills. Cabs in New York, I’m certain, obey the speed limit.
Use a comma in certain conventional situations (to separate items in dates and addresses, after the salutation and closing of a letter, and after a name followed by a title).
Examples: January 1, 1992 New York, NY Dear Shirley, Cordially, Albert Schweitzer, Ph.D.
Note 1: One bracketing comma will suffice if the weak interruption comes at the beginning or the end of the sentence.
[Although often wet, Britain has lots of sunshine. as opposed to Britain, although often wet, has lots of sunshine.]
Note 2: The main purpose of punctuation is to aid understanding; a subsidiary purpose is to aid flow. Use joining commas and pairing commas where this aids understanding and/or flow. As a general rule, the longer the sentence or the more complex the sentence, the greater the need for commas.
Note 3: When in doubt over where to use a comma, try reading the sentence out loud and, generally speaking, commas should be used where you pause for clarification or breath.
Note 4: There is some controversy over use of something called the serial or Oxford comma which is the last comma in this example: The colours in the Union Jack flag are red, white, and blue. Generally the serial comma is not used in Britain where it is regarded as unnecessary, but it is commonly used in the United States where it is thought helpful. However, many authors prefer to use a listing comma before ‘and’ or ‘or’ only when it is necessary to make the meaning clear.
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