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:
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.
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