essential vs. nonessential phrases
A friend of mine, George, called the other day.
The poem "The Road Not Taken" is one of Robert Frost's most famous works.
How come sentence 1 requires commas and sentence 2 does not? Both phrases ("George" and "The Road Not Taken") both specify the subject.
"Rule: When an appositive is essential to the meaning of the noun it belongs to, don’t use commas. When the noun preceding the appositive provides sufficient identification on its own, use commas around the appositive.
"Example: Jorge Torres, our senator, was born in California. Explanation: Our senator is an appositive of the proper noun Jorge Torres. Our senator is surrounded by commas because Jorge Torres is a precise identifier" (from here).
Other examples:
- My brother Ken is a minister in the PCA denomination.
- My trainer, Jim Shipshape, is also a bodybuilder.
- Raconteur Ed Schlemiel will be speaking at the Jewish Community Center tonight.
- Mezzo-soprano Hilda Gutenberg is a world-class opera singer.
- Question: Who is coming with you on the canoe trip, and what is his name? Answer: My dad Harold will be accompanying me.
In conclusion, the appositive in your first sentence is George. Does the noun phrase which precedes it provide sufficient identification on its own? The answer is yes. So set off the appositive with commas.
The appositive in your second sentence is "The Road Not Taken." Are the words which precede it (namely, the poem) essential to its meaning? The answer is yes, because Frost wrote many, many poems, but only one of his poems is most famous. Therefore, the poem and "The Road Not Taken" are essential to one another. No commas are needed.
- A friend of mine, George, called the other day.
- The poem "The Road Not Taken" is one of Robert Frost's most famous works.
are, respectively, reduced by Whiz-Deletion from relative clauses
- A friend of mine, who is (named) George, called ...
- The poem which/that is (entitled) "The Road Not Taken" is one of ...
The relative clause in (3) is a nonrestrictive clause, with comma intonation on either end,
while the one in (4) is a restrictive clause (the kind that allows that instead of which), and
no special intonation or punctuation prescribed.
Either kind of relative clause produces an appositive NP whenever the clause has a predicate noun, and its subject and auxiliary be are removed by Whiz-Deletion. That means there are two kinds of apposition, with the same senses as restrictive and non-restrictive relatives.
Similarly, Whiz-Deletion produces postnominal modifier phrases with other types of predicates that use be auxiliaries:
- A tree uprooted by the storm <== A tree that was uprooted by the storm
- The man standing on the corner <== The man who is/was standing on the corner
- Somebody angry at the authorities <== Somebody who is/was angry at the authorities