The Elements of Style · Rule 3

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 be slight or considerable, he must never insert one comma and omit 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.

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.

Always to be regarded as parenthetic and to be enclosed between commas (or, at the end of the sentence, between comma and period) are the following:

(1) the year, when forming part of a date, and the day of the month, when following the day of the week:

• February to July, 1916.
• April 6, 1917.
• Monday, November 11, 1918.

(2) the abbreviations etc. and jr.

(3) non-restrictive relative clauses, that is, those which do not serve to identify or define the antecedent noun, and similar clauses introduced by conjunctions indicating time or place.

• The audience, which had at first been indifferent, became more and more interested.

In this sentence the clause introduced by which does not serve to tell which of several possible audiences is meant; what audience is in question is supposed to be already known. The clause adds, parenthetically, a statement supplementing that in the main clause. The sentence is virtually a combination of two statements which might have been made independently:

• The audience had at first been indifferent. It became more and more interested.

Compare the restrictive relative clause, not set off by commas, in the sentence,

• The candidate who best meets these requirements will obtain the place.

Here the clause introduced by who does serve to tell which of several possible candidates is meant; the sentence cannot be split up into two independent statements.

The difference in punctuation in the two sentences following is based on the same principle:

• Nether Stowey, where Coleridge wrote _The Rime of the Ancient
  Mariner_, is a few miles from Bridgewater.
• The day will come when you will admit your mistake.

Nether Stowey is completely identified by its name; the statement about Coleridge is therefore supplementary and parenthetic. The day spoken of is identified only by the dependent clause, which is therefore restrictive.

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.

• 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, exchanged afterwards for Sardinia.

Other illustrations may be found in sentences quoted under Rules 4, 5, 6, 7, 16, and 18.

The writer should be careful not to set off independent clauses by commas: see under Rule 5.