Page:The Elements of Style (Strunk).djvu/13

 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.

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.