COMMAS IN A COMPOUND SENTENCE. It's a balancing act.

Guideline. Use a comma between independent clauses separated by a conjunction (and, or, for, but, etc.) in a compound sentence. Use your judgment when the clauses are short or the subject is the same for both clauses.

• Our group has spent the past sixteen months studying the effect that solar storms have on editing ability, and our manager will be delivering her report next Friday. (A comma separates the two clauses because each clause represents an independent thought.)

• I hate the idea, and Sassafras feels the same way. (A comma separates the two clauses because each clause represents an independent thought.)

A closer look. If the two independent clauses in a compound sentence are short and, in particular, if the subject is the same in both, the comma becomes less critical. In general, though, it's a good idea to follow the rule no matter what. This will prevent you from writing sentences that read as follows:

• I hate the idea and Sassafras feels the same way.

• I intend to eliminate the problem and Brutus has promised to help.

CLAUSES AND PHRASES IN CONTRAST: Accentuating the negative

Guideline. Use a comma to give more emphasis to a clause or phrase that contradicts or draws a contrast to an earlier idea.

• We introduced the new double-seat air chair because we believed in it, not because we simply wanted to add another model to our line. (Note: If you wanted to give even more emphasis to the clause in italics, you could have used a dash.)

INTERRUPTORS: Changing direction

Guideline. Use a comma to set off short words and phrases that interrupt the flow of the sentence, especially when they contradict, qualify, or amend what has come before.

• We would love to compete in your tournament. We have just learned, however, that we must be in Sheboygan that weekend. (Commas are needed because however interrupts the flow of the sentence.

• The new course, in contrast to the course laid out last summer, is in a much smoother part of the lake. (Commas are needed here because the phrase in contrast to the course laid out last summer interrupts the flow of the sentence.

A closer look. If you want to deemphasize the pauses that would normally occur as the result of a phrase that interrupts the flow, you can omit the commas.

• We would love to compete in your tournament. We were planning in fact to hold a tournament of our own. (Omitting the commas before and after in fact makes the sentence flow more smoothly and quickly takes the emphasis away from in fact.

Source: Grammar for Smart People, by Barry Tarshis.

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