The Elements of Style, Fourth Edition

by William Strunk Jr.

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: