A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Comma - Independent clauses
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Part III. Style 20 spelling

Commas separate items within a sentence, including clauses, phrases, and individual words. They are especially important when a reader might mistake where a clause or phrase ends and another begins:

Before leaving the members of the committee met in the assembly room.

Before leaving, the members of the committee met in the assembly room.

For use of commas in numbers, see 23.2.2. For use of commas in citations, see 16.1.2 and 18.1.2.

21.2.1 Independent clauses

In a sentence containing two or more independent clauses joined by a coordinating conjunction (and, but, or, nor, for, so, yet), put a comma before the conjunction. This is not a hard-and-fast rule; no comma is needed between two short independent clauses with no internal punctuation.

Students around the world want to learn English, and many young Americans are eager to teach them.

The senator arrived at noon and the president left at one.

In a sentence containing three or more short and simple independent clauses with no internal punctuation, separate the clauses with commas and add a coordinating conjunction before the last one. (Always include a comma before the coordinating conjunction.) If the clauses are longer and more complex, separate them with semicolons (see 21.3)—or, better, rewrite the sentence.

The committee designed the questionnaire, the field workers collected responses, and the statistician analyzed the results.

The committee designed the questionnaire, which was short; the field workers, who did not participate, collected responses; and the statistician analyzed the results, though not until several days later.

Ordinarily, do not insert a comma before a conjunction joining two subjects or two predicates.

The agencies that design the surveys and the analysts who evaluate the results should work together.

They do not condone such practices but attempt to refute them theoretically.

When a sentence opens with a phrase or dependent clause that modifies following two independent coordinate clauses, put a comma after the introductory element but not between the two independent clauses.

Within ten years, interest rates surged and the housing market declined.