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“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 commas may be safely omitted. But whether the interruption is slight or considerable, never omit one comma and leave the other. There is no defense of such punctuation as

Marjorie’s husband, Colonel Nelson paid us a visit yesterday.

or

My brother, you will be pleased to hear, is now in perfect health,

Dates usually contain parenthetic words of figures. Punctuation is as follows:

February to July, 1992

April 6, 1986

Wednesday, November 14, 1990

Note that it is customary to omit the comma in

6 April 1988

The last form is an excellent way to write a date; the figures are separated by a word and are, for that reason, quickly grasped.

A name of title in direct address in parenthetic.

If, Sir, you refuse, I cannot predict what will happen.

Well, Susan, this is a fine mess your are in.

The abbreviations etc., i.e., and e.g., the abbreviations of academic degrees, and titles that follow a name are parenthetic and should be punctuated accordingly.

Letters, packages, 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

The novelist Jane Austen

William the Conquerer

The poet Sappho

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 or 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 and 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. In the first example, the clause introduced by which does not serve to tell which of several possible audiences is meant; the reader presumably knows that already. The clause adds, parenthetically, a statement supplementing that in the main clause. Each of the three sentences is a combination of two statements that might have been made independently.

The audience was at first indifferent. Later it became more and more interested.

Napoleon was born in 1769. At that time Corsica had but recently been acquired by France.

Coleridge wrote The Rime of the Ancient Mariner at Nether Stowey. Nether Stowey is a few miles from Bridgewater.

Restrictive clauses, by contrast, are not parenthetic and are not set off by commas. Thus,

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 sentence above, cannot be split into two independent statements. The same principle of comma use applies to participial phrased and to appositives.

People sitting in the rear couldn’t hear. (restrictive)

Uncle Burt, being slightly deaf, moved forward, (non-restrictive)

My cousin Bob is a talented harpist. (restrictive).

Our oldest daughter, Mary, sings. (non-restrictive)

When a the main clause of a sentence is preceded by a phrase or a subordinate clause, us a comma to set off these elements.

Partly by hard fighting, partly by diplomatic skill, they enlarged their dominions to the east and rose to royal rank with the possession of Sicily.”

Excerpted from: Strunk, William Jr., and E.B. White. The Elements of Style, Fourth Edition. New York: Longman, 2000.