The Elements of Style. Strunk William
(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.
In this sentence the clause introduced by which does not serve to tell which of several possible audiences is meant; what audience is in question is supposed to be already known. The clause adds, parenthetically, a statement supplementing that in the main clause. The sentence is virtually a combination of two statements which might have been made independently:
The audience had at first been indifferent. It became more and more interested.
Compare the restrictive relative clause, not set off by commas, in the sentence,
The candidate who best meets these requirements will obtain the place.
Here the clause introduced by who does serve to tell which of several possible candidates is meant; the sentence cannot be split up into two independent statements.
The difference in punctuation in the two sentences following is based on the same principle:
Nether Stowey, where Coleridge wrote The Rime of the Ancient Mariner, is a few miles from Bridgewater.
The day will come when you will admit your mistake.
Nether Stowey is completely identified by its name; the statement about Coleridge is therefore supplementary and parenthetic. The day spoken of is identified only by the dependent clause, which is therefore restrictive.
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.
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, exchanged afterwards for Sardinia.
Other illustrations may be found in sentences quoted under Rules 4, 5, 6, 7, 16, and 18.
The writer should be careful not to set off independent clauses by commas: see under Rule 5.
4. Place a comma before a conjunction introducing a co-ordinate 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 afterthought. 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 the second member is introduced by an adverb, a semicolon, not a comma, is required (see Rule 5). The connectives so and yet may be used either as adverbs or as conjunctions, accordingly as the second clause is felt to be co-ordinate or subordinate; consequently either mark of punctuation may be justified. But these uses of so (equivalent to accordingly or to so that) are somewhat colloquial and should, as a rule, be avoided in writing. A simple correction, usually serviceable, is to omit the word so and begin the first clause with as or since:
| I had never been in the place before; so I had difficulty in finding my way about. | As I had never been in the place before, I had difficulty in finding my way about. |
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.
When the subject is the same for both clauses and is expressed only once, a comma is required if the connective is but. If the connective is and, the comma should be omitted if the relation between the two statements is close or immediate.
I have heard his arguments, but am still unconvinced.
He has had several years’ experience and is thoroughly competent.
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.
It is nearly half past five, and we cannot reach town before dark.
A comparison of the three forms given above will show clearly the advantage of the first. It is, at least in the examples given, better than the second form, because it suggests the close relationship between the two statements in a way that the second does not attempt, and better than the third, because briefer and therefore more forcible. Indeed it may be said that this simple method of indicating relationship between statements is one of the most useful devices of composition. The relationship, as above, is commonly one of cause or of consequence.
Note that if the second clause is preceded by an adverb, such as accordingly, besides, then, therefore, or thus, and not by a conjunction, the semicolon is still required.
Two exceptions to the rule may be admitted. If the clauses are very short, and are alike in form, a comma is usually permissible :
Man proposes, God disposes.
The gate swung apart, the bridge fell, the portcullis was drawn up.
Note that in these examples the relation is not one of cause or consequence. Also in the colloquial form of expression,
I hardly knew him, he was so changed,
a comma, not a semicolon, is required. But this form of expression is inappropriate in writing, except in the dialogue of a story or play, or perhaps in a familiar letter.
6.