I was recently editing a memoir for a client and I had some questions about comma usage. I found an article on the internet that explained everything I needed to know. I am constantly amazed at how easy it is to get the information you need with just a few clicks of the mouse. I have decided to reprint the entire article here. To tell the truth, I did this mostly for me so I can have quick access.
A Reprint from: The Center for Writing Studies
The comma is important because it is the main device by which the grouping of words, phrases, and clauses is indicated. Consequently, it is used and unfortunately misused, more than all the other marks combined. Its use, however, is not haphazard. Competent writers almost always use commas for one of two purposes: to set off some element of the sentence from what precedes, what follows, or both, or else to separate two elements as they might be separated by a pause or rising inflection of the voice if one were speaking. The few uses that fall into neither of these categories are the arbitrary use of commas on certain specific occasions, to be pointed out below, and the insertion of a comma when its presence is necessary for the sake of clarity.
The rules that follow, though numbered consecutively throughout, are grouped in accordance with the uses referred to above:
Commas Used to Set Off
An appositive or a term of direct address is set off by commas:
The original factory, an old stone structure, is still standing.
An adverbial clause preceding its principal clause, or an adverbial phrase at the beginning of a clause, is usually set off by a comma:
On all floors except the second and the fourth, the fire hazards have been removed.
Note: If an adverbial clause or phrase is extremely short, and if the omission of the comma could not cause confusion, the comma may be omitted. For example:
When he arrived he was immediately seated
During July the plant will be closed.
Independent elements, participial phrases, gerund phrases, and other such constructions at the beginning of a sentence are set off by commas:
No, the shipment has not yet arrived
Worried by the complaints, we began an investigation.
A conjunctive adverb (however, moreover, therefore, etc.) is usually set off by commas when it comes within the clause to which it applies. When it comes at the beginning of a clause, it may or may not be followed by a comma but will always be preceded by a period or semicolon:
His objection, therefore, was ignored -or- I had heard the rumor before; consequently, I did not believe it.
Any mildly parenthetical element is enclosed in commas if it seems desirable to set it apart from the rest of the sentence. A writer is called upon to use his own judgment in applying this rule, for too many commas will make a sentence jerky and hard to read:
The frame, he insisted, was too tight.
A term such as “namely” or “that is,” used to introduce an example or a list, is usually set apart from that example or list by a comma. (The mark that precedes such an expression depends on the sentence structure.)
Three species of trees were observed, namely, pine, fir, and cedar.
Nonrestrictive clauses are set off by commas. Restrictive clauses, however, are not set off:
The south side, which had been exposed to the sun, was badly faded.
He moved to Arizona, where the climate was not so moist.
Note: In these examples, the clauses introduced respectively by “which” and “where” merely add some additional facts. If they were omitted, the meaning of the remainder of the sentence would be unchanged. Hence they are nonrestrictive. However, some clauses introduced by “where” or “who” are restrictive. Each is used to limit–to restrict–the meaning of the main statement, which would be radically changed if the clause in question were omitted.
The roads went to pieces where the permafrost had been disturbed.
All motorists who drive recklessly should be fined heavily.
Sometimes a sentence does not make sense unless a clause is interpreted in a single way–restrictive or nonrestrictive. When this is true, an error in punctuation merely increases the difficulty of reading. There are times, however, when restrictive and nonrestrictive interpretations are equally reasonable. When this is the case, an error in punctuation leads a reader to misunderstand the meaning. Note how the meaning of the two sentences that follow depends on punctuation:
The people from Troy, who had come early, obtained seats.
The people from Troy who had come early obtained seats.
A word or phrase placed in an abnormal position in a sentence should be set off by a comma or commas:
To a trained accountant, the problem would look easy.
A direct quotation is set off by a comma or commas:
“The tires are threadbare,” he asserted, “and will blow out at any moment.”
Exceptions: A quotation that blends into the regular structure of the sentence is not set off by commas. A title in quotation marks is not set off by commas unless some other rule makes commas necessary:
The poet’s prophecy about “airy navies grappling in the central blue” has become an unpleasant reality.
The rhythm of “The Raven” is very striking.
Commas Used to Separate
A comma is ordinarily used between two independent clauses that are joined by a coordinating conjunction. The coordinating conjunctions are “and,” “but,” “for,” “or,” and “nor.” (“Yet” and “so” may also be treated as coordinating conjunctions when this rule is applied.)
The building is old, but it has been kept in good condition.
Note: When both clauses are extremely short and simple, the comma may be omitted:
It was damaged but it still is usable.
Note: If a comma is used within one or both of two independent clauses, the comma between them is sometimes replaced by a semicolon.
When a sentence contains a series, the elements in the series are normally separated by commas.
Cattle, sheep, and hogs are now selling for higher prices.
Note: If a comma is used within any element in a series, it is often better to use semicolons rather than commas between the elements:
We visited Paris, Cannes, and Avignon in France; Frankfurt, Bonn, and Berlin in Germany; and Madrid and Seville in Spain.
Note: Opinions differ over whether to use a comma before a conjunction (“and” or “or”) that precedes the last item in a series. In technical and scientific periodicals and in the material published by the United States Government, the use of the comma is predominant. In journalistic and popular publications, usage is divided. Sometimes a comma is essential for clarity because of “and” or “or” being used within one of the items. For example:
The panels were painted red, green, yellow, and black and white.
Without the comma after “yellow,” it would be impossible to know whether “black” belonged with “yellow” or with “white.” In view of this, it seems advisable to regard the comma as normal punctuation rather than trying to check each series to see whether a comma is needed for clarity.
Two or more adjectives preceding a noun are ordinarily separated by commas. (The comma before the last adjective is omitted, however, if that adjective is so closely associated with the noun that the two merge into a single thought unit.) Also, a comma is used between adverbs that modify the same object.
He has a modest, unassuming manner.
The watchman was a feeble old man.
Slowly, relentlessly, the stream wore away the dike.
Commas are variously used to separate items in dates, places, and numbers.
In dates: He was born on December 4, 1963, in Princeton.
In places and addresses: San Francisco, California, is an important shipping point.
To separate adjacent sets of figures: In 1950, 675 men were added to the payroll.
Between the digits of numbers: 10,984; 234,617; and 1,856,445.
Note: In many cases, the comma is omitted in a number with only four digits, unless the number occurs in a column containing numbers in which commas are used
Commas Used Arbitrarily
A comma is used between the last and the first name of a person when the last name appears first, and also after the first name unless the sentence structure calls for some other mark:
Please insert the name Fitzgerald, Duane, in the proper place in the alphabetical list.
A comma may be used whenever it is necessary to force a pause for the sake of clarity.
Inside, the building was in better condition.