Here is the next section of the first part of Strunk and White's Elements of Style: ELEMENTARY RULES OF USAGE.
"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 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 for such punctuation as
Marjorie's husband, Colonel Nelson paid us a visit yesterday.
My bother you will be pleased to hear, is now in perfect health.
Dates usually contain parenthetic words or figures. Punctuate 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 or a title in direct address is parenthetic.
If, Sir, you refuse, I cannot predict what will happen.
Well, Susan, this is a fine mess you are in.
The abbreviations etc., i.e., and e.g., the abbreviations for 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 Conqueror
The poet Sappho
Although Junior, with its abbreviation Jr., has commonly been regarded as parenthetic, the logic suggests that it is, in fact, restrictive and therefore not in need of a comma.
James Wright Jr."
(to be continued next week)