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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,

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 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.

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.

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:

Or the subordinate clauses might be replaced by phrases:

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:

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.

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.

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.

It is of course equally correct to write the above as two sentences each, replacing the semicolons by periods.

If a conjunction is inserted the proper mark is a comma (Rule 4).

A comparison of the three forms given above will show clearly the advantage of the first. It is, at least in the