When a coordinating conjunction connects two or more independent clauses—word groups that could stand alone as separate sentences—a comma must precede the conjunction. There are seven coordinating conjunctions in English: and, but, or, nor, for, so, and yet.

A comma tells readers that one independent clause has come to a close and that another is about to begin.

exception: If the two independent clauses are short and there is no danger of misreading, the comma may be omitted.

tip: As a rule, do not use a comma with a coordinating conjunction that joins only two words, phrases, or subordinate clauses. (See 33a. See also 32c for commas with coordinating conjunctions joining three or more elements.)

The word group following and is not an independent clause; it is the second half of a compound predicate (controls . . . and invests).