This is an article from a forum post on thenextbigwriter.com, a great writing website. Thank you, crazeesharon!
Sources (1) Harold Allen; (2) William Strunk Jr. & E.B. White; (3) Susan Thurman; (4) Bonnie Trenga.
LESSON TWO: THE COMMA
(1) Although most of its uses are conventional, the comma responds more sensitively than other punctuation marks to the needs of the individual writer. Its use or nonuse in certain situations enables a writer to communicate subtle meanings and relationships not readily shown in words alone.
The structural uses of the comma usually correlate in speech with the level or the rising intonation accompanied by pause, and in writing with definable grammatical structures.
A major structural use of the comma is to set off certain language units (dialogue) from the rest of the sentence. If any such unit is at the beginning or the end of a sentence, usually one comma is needed; if it is within the sentence, then usually two commas are needed.
Use commas to set off nonrestrictive elements. A nonrestrictive element is one that adds information without limiting or cutting down the idea of the main clause. It may be identified by your supplying the testing clue, "and by the way," as well as by the pause following level or rising intonation.
Use commas to set off a nonrestrictive clause:
William James, who was a pioneer in pyschology, developed the philosophical idea of pragmatism. ( "and by the way" -- who was a pioneer in pyschology)
But not a restrictive clause:
Only staff members who are teachers will go to the convention.
Use commas to set off a nonrestrictive phrase:
It was also raining here when the senator's plane landed, at the end of his trip.
But not a restrictive phrase:
Other members of the senator's party were making different kinds of news on his first morning back.
Use commas to set off that special nonrestrictive word or phrase usually called an appositive:
I called the press officer, David Kelsey.
Do not use commas with close apposition, which is not marked by the intonation plus pause:
I called press agent David Kelsey.
Use commas to set off a negative appositive:
It is paid for in local currency, not gold.
Use commas to set off parenthetical words and phrases:
Had he been back home, of course, he would have gone by jet or car.
Use commas to set off transitional words and phrases:
Nevertheless, man still has not learned to fly like a bird.
Consider, for example, the invention of one of the world's all-time best-selling games.
But not when a transitional word or phrase is an integral part of the sentence:
The next man to speak was a local politician.
Use commas to set off an adverbial clause at the beginning or in the middle of a sentence:
It also seems very likely that, where obscenity laws continue to be constitutionally applicable, the Supreme Court will give the law relatively broad scope.
However, a short introductory clause (element) may occur without the comma if the meaning is quite clear, as when both clauses have the same subject:
When he saw the sun he shouted for joy.
Use commas to set off an adverbial phrase out of its expected position after the verb:
The doors slid silently shut, and, without so much as a slight jerk, the train sped away.
Use commas to set off words in direct address:
"Can you spare him for a few minutes, Shirley?"
Use commas to prevent ambiguity or misunderstanding:
What is needed are investment funds to start up the service network, and assured means for paying for continuing home service.
(The comma prevents the reader from taking both "network" and "means" as objects of "start up.")
Commas are often used to separate coordinate or correlative sentence parts, especially when such separation is indicated in speech by level or rising intonation pattern plus pause.
Except when the clauses are short and the meaning quite clear, use a comma before a conjunction that precedes the second clause of a compound sentence. The comma is most necessary before FOR:
It's very hard work, for the children led quite pampered lives up till now.
But not in:
Karla withdrew the doll and the child released it with a sigh.
Between short main clauses you may use a comma without a conjunction when it represents a rising or level intonation with pause. Note that this use is not the objectionable "sentence fault" or "comma splice" that occurs with a falling intonation:
Decor is part of the dining experience. People like it, people demand it.
She left early, she had such a bad headache.
Use commas to separate words, phrases, and clauses that constitute a series:
I had a lunch of sauerbraten, red cabbage, lentils, pumpernickel, and dark beer.
Newspapers and some other publications tend to omit the comma before AND on the assumption that the comma represents the absence of a conjunction and thus is not needed when the conjunction is present. Actually, it represents a perceptible rising or level intonation pattern with pause, which occurs before the final member of the series regardless of the presence or absence of AND. It is best to use a comma there consistently, so that inadvertently you do not produce such a result as this:
Berle can sing, dance, juggle, act, do card tricks, imitations and acrobatics, ride a unicycle and mug underwater.
You may use a comma to separate long predicative constructions even when a conjunction is present:
Even the newest bones were now crusted with moss and lichen, and were scarred with the teeth mark of foxes.
Use a comma to separate two or more adjectives that independently modify a noun, as would be signaled by an intonation pattern with pause:
Please enclose a stamped, self-addressed envelope.
Compare a "bright blue dress" to a "bright, blue dress." Without the comma, bright modifies the color blue. With the comma, both bright and blue describe the dress.
Use a comma to indicate the omission of an element that, if present, would parallel the same element in a preceding part of the sentence:
Most of the young climbers had come by motor scooters; the older ones, by train.
Certain uses of the comma are nonstructural -- arbitrary and mechanical rather than related to the syntax.
Use a comma to set off a direct quotation:
"I eat too much," Adler confessed.
Use commas to separate place names:
Bernard came from Astoria, Oregon.
No punctuation mark is used between the offical post office abbreviation for a
state and a zip code number. Cheyenne, WY 82003
Use commas in dates:
On September 3, 1939, the judge offered him the choice between prison and
There is a tendency not to use the comma when the date of the month is not given, and also to omit it after the year even if the date is given:
Steller and his companions abandoned ship on Bering Island in November 1741.
(2) In a series of three or more terms with a single conjunction, use a comma after each term except the last:
red, white, and blue (newspaper style eliminates the comma between white and blue)
Enclose parenthetic expressions between commas:
The best way to see a country, unless you are pressed for time, is to travel on foot.
This rule is difficult to apply; it is frequently hard to decide whether a single word, such as HOWEVER, or a brief phrase is or is not parenthetic. If the interruption to the flow of the sentence is but slight, the writer may safely omit the commas. But whether the interruption is slight or considerable, he must never omit one comma and leave the other.
A name or a title in direct address is parenthetic:
If, Sir, you refuse, I cannot predict what will happen.
The abbreviations etc., i.e., and e.g., the abbreviations for academic degrees, and titles that follow a name are parenthetic and should be punctuated accordingly:
Letters, packaged, etc., should go here.
Horace Fulsome, Ph.D., presided.
Rachel Simonds, Attorney
The Reverend Harry Lang, S.J.
No comma, however, should separate a noun from a restrictive term of identification:
Billy The Kid
William The Conqueror
Although JUNIOR, with its abbreviation JR., has commonly been regarded as parenthetic, logic suggests that it is, in fact, restrictive and therefore not in need of a comma:
James Wright Jr.
Nonrestrictive relative clauses are parenthetic, as are similar clauses introduced by conjunctions indicating time and place. Commas are therefore needed. A nonrestrictive clause is one that does not serve to identify or define the antecedent noun:
The audience, which had at first been indifferent, became more interested.
In 1769, when Napoleon was born, Corsica had but recently been acquired by France.
Nether Stowey, where Coleridge wrote "The Rime of the Ancient Mariner," is a few miles from Bridgewater.
In these sentences, the clauses introduced by WHICH, WHEN and WHERE are nonrestrictive; they do not limit or define, they merely add something.
Restrictive clauses, by contrast, are not parenthetic and are not set off by commas:
People who live in glass houses shouldn't throw stones.
Here the clause introduced by WHO does serve to tell which people are meant; the sentence, unlike the sentences above, cannot be split into two independent statements.
The same principle of comma use applies to participial phrases and to appositives:
People sitting in the rear couldn't hear. (restrictive)
Uncle Bert, being slightly deaf, moved forward. (nonrestrictive)
When the main clause of a sentence is preceded by a phrase or a subordinate clause, use a comma to set off these elements:
Partly by hard fighting, partly by diplomatic skill, they enlarged their dominions to the east.
Place a comma before a conjunction introducing an independent clause:
The early records of the city have disappeared, and the story of its first years can no longer be reconstructed.
Two-part sentences of which the second member is introduced by AS (in the sense of "because"), FOR, OR, NOR or WHILE (in the sense of "and at the same time") likewise require a comma before the conjunction.
If a dependent clause, or an introductory phrase requiring to be set off by a comma, precedes the second independent clause, no comma is needed after the conjunction:
The situation is perilous, but if we are prepared to act promptly, there is still one chance of escape.
When the subject is the same for both clauses and is expressed only once, a comma is useful if the connective is BUT. When the connective is AND, the comma should be omitted if the relation between the two statements is close or immediate:
I have heard his arguments, but am still unconvinced.
He has had several years' experience and is thoroughly competent.
Do not join independent clauses by a comma. If two or more clauses grammatically complete and not joined by a conjunction are to form a single compound sentence, the proper mark of punctuation is the semicolon.
An exception to the semicolon rule: a comma is preferable when the clauses are very short and alike in form, or when the tone of the sentence is easy and conversational:
Man proposes, God disposes.
The gates swung apart, the bridge fell, the porcullis was drawn up.
Do not break sentences in two. Do not use periods for commas:
I met them on a Cunard liner many years ago. Coming home from Liverpool to New York.
The period should be replaced by a comma. It is permissible to make an emphatic word or expression serve the purpose of a sentence and to punctuate it accordingly:
Again and again he called out. No reply.
The writer must, however, be certain that the emphasis is warranted, lest his clipped sentence seem merely a blunder in syntax or in punctuation. Generally speaking, the place for broken sentences is in dialogue, when a character happens to speak in a clipped or fragmented way.