Elementary Rule 3. Enclose parenthetic expressions between commas in The Elements of Style
e.g. : 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. （这个原则使用的难点在于判断一个词，或词组，或从句是否属于附加说明）
不过，决不能只使用一个逗号。
e.g. : Marjories husband, Colonel Nelson paid us a visit yesterday.
作者罗列了一些属于和不属于附加性说明的情况：
1. A name or a title in direct address is parenthetic. （确切的名字或称谓）
e.g. : If, Sir, you refuse, I cannot predict what will happen.
Well, Susan, this is a fine mess you are in.
2. 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.
e.g.: Letters, packages, etc., should go here.
Horace Fulsome, Ph.D., presided.
Rachel Simonds, Attorney
The Reverend Harry Lang, S.J.
3. No comma, however, should separate a noun from a restrictive term of identification. （不要用逗号分隔关于身份的限定性说明）
e.g.: Billy the Kid
The novelist Jane Austen
William the Conqueror
The poet Sappho
4. Nonrestrictive relative clauses are parenthetic, as are similar clauses introduced by conjunctions indicating time or place. Commas are therefore needed. A nonrestrictive clause is one that does not serve to identify or define the antecedent noun. （非限定性从句属于附加说明，需要用前后两个逗号把它分隔出来）
e.g: The audience, which had at first been indifferent, became more and more interested.
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 nonrestrictive; they do not limit or define, they merely add something. (在上述例子中，从句添加了一些独立的信息，所以它们不属于限定性表达)
它们可以变成两个独立的句子:
e.g.: The audience was at first indifferent. Later it became more and more interested.
5. Restrictive clauses, by contrast, are not parenthetic and are not set off by commas. （限定性从句则无需用逗号将其分隔出来）
e.g: People who live in glass houses shouldn't throw stones.
这个例子中的从句属于限定性表达，无法将从句和主语写成两个独立的句子。这个原则同样适用于分词词组或同位语表达。
e.g. People sitting in the rear couldn't hear, (restrictive)
Uncle Bert, being slightly deaf, moved forward, (non-restrictive)
My cousin Bob is a talented harpist, (restrictive)
Our oldest daughter, Mary, sings, (nonrestrictive)
6. When the main clause of a sentence is preceded by a phrase or a subordinate clause, use a comma to set off these elements. （当主句之前有一个词组，或附属从句，用逗号分隔）
e.g. 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.