Enclose Parenthetic Expressions Between Commas

ELEMENTARY RULES OF USAGE

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 be slight or considerable, he must

never omit one comma and leave the other. 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,

is indefensible.

Non-restrictive relative clauses are, in accordance with this

rule, set off by commas.

The audience, which had at first been indifferent, became

more and more interested.

Similar clauses introduced by where and when are similarly

punctuated. 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 non-restrictive; they do not limit the application

of the words on which they depend, but add, parenthetically,

statements supplementing those in the principal clauses. Each

sentence is a combination of two statements which 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 only a few miles from

Bridgewater.

Restrictive relative clauses are not set off by commas.

The candidate who best meets these requirements will

obtain the place.

Series of three or more Terms with a Single Conjunction, use a Comma after each term except the Last

In this sentence the relative clause restricts the application

of the word candidate to a single person. Unlike those above,

the sentence cannot be split into two independent statements.

The abbreviations etc. and jr. are always preceded by a comma, and except at the end of a sentence, followed by one.

Similar in principle to the enclosing of parenthetic

expressions between commas is the setting off by commas of

phrases or dependent clauses preceding or following the main

clause of a sentence. The sentences quoted in this section and

under Rules 4, 5, 6, 7, 16, and 18 should afford sufficient

guidance.

If a parenthetic expression is preceded by a conjunction,

place the first comma before the conjunction, not after it.

He saw us coming, and unaware that we had learned of

his treachery, greeted us with a smile.

Source: The Elements of Style by William Strunk, Jr.

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