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 omit one comma and leave the other. Such punctuation as
Marjorie’s husband, Colonel Nelson paid us a visit yesterday,
My brother you will be pleased to hear, is now in perfect health,
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.
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.