Non-restrictive relative clauses are, in accordance with this rule, set off by commas. (来源：英语麦当劳www.EnglishCN.com)
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 statments 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.
4. Place a comma before and or but introducing an independent clause.
The early records of the city have disappeared, and the story of its first years can no longer be reconstructed.
The situation is perilous, but there is still one chance of escape.
Sentences of this type, isolated from their context, may seem to be in need of rewriting. As they make complete sense when the comma is reached, the second clause has the appearance of an after-thought. Further, and, is the least specific of connectives. Used between independent clauses, it indicates only that a relation exists between them without defining that relation. In the example above, the relation is that of cause and result. The two sentences might be rewritten:
As the early records of the city have disappeared, the story of its first years can no longer be reconstructed.
Although the situation is perilous, there is still one chance of escape.
Or the subordinate clauses might be replaced by phrases:
Owing to the disappearance of the early records of the city, the story of its first years can no longer be reconstructed.
In this perilous situation, there is still one chance of escape.
But a writer may err by making his sentences too uniformly compact and periodic, and an occasional loose sentence prevents the style from becoming too formal and gives the reader a certain relief. Consequently, loose sentences of the type first quoted are common in easy, unstudied writing. But a writer should be careful not to construct too many of his sentences after this pattern (see Rule 14).
Two-part sentences of which the second member is introduced by as (in the sense of because), for, or, nor, and while (in the sense of and at the same time) likewise require a comma before the conjunction.
If a dependent clause, or an introductory phrase requiring to be set off by a comma, precedes the second independent clause, no comma is needed after the conjunction.
The situation is perilous, but if we are prepared to act promptly, there is still one chance of escape.
For two-part sentences connected by an adverb, see the next section.
5. Do not join independent clauses by a comma.
If two or more clauses, grammatically complete and not joined by a conjunction, are to form a single compound sentence, the proper mark of punctuation is a semicolon.
Stevenson's romances are entertaining; they are full of exciting adventures.
It is nearly half past five; we cannot reach town before dark.
It is of course equally correct to write the above as two sentences each, replacing the semicolons by periods.
Stevenson's romances are entertaining. They are full of exciting adventures.
It is nearly half past five. We cannot reach town before dark.
If a conjunction is inserted, the proper mark is a comma (Rule 4).
Stevenson's romances are entertaining, for they are full of exciting adventures.