Student's guide to writing college papers, Fourth edition - Kate L. Turabian 2010

Introductory elements
Punctuation
Style

Sentences often begin with an introductory word, phrase, or subordinate clause before the main clause begins. When they are short these introducers pose little problem for readers, as in this sentence. But when (as in this sentence) an introducer becomes long enough that readers cannot keep it all in mind at once, especially if it includes multiple elements, readers need punctuation to help them keep the grammatical units straight in their minds. We can give you a few rules to guide you in punctuating introductory elements, but in many cases you will have to rely on your judgment to decide what readers need.

22.3.1 Summary

Use a comma to set off the following introductory elements:

1. Connecting adverb or adverb phrase (with some exceptions)

2. Commenting adverb or adverb phrase

3. Long introductory phrase or clause

4. Introductory element that might confuse readers

22.3.2 Four Ways to Punctuate Introductory Elements

1. Put a comma after an initial adverb or adverb phrase that connects the current sentence to previous ones.

Connecting adverbs include such terms as however, nevertheless, meanwhile, also, in addition, therefore, and so on. Since readers tend to hear a mental pause after these terms, they usually expect a comma.

In the meantime, the police were distracted by a noise in the alley. Conversely, binge drinkers tend to underestimate the risks of excessive drinking.

Exception: You may omit the comma after a connecting adverb if it does not create a pause when you read aloud, especially for short terms like now, thus, hence, perhaps, and so on.

Perhaps we will see you there.

Now the evidence supports no such conclusion.

2. Put a comma after an initial adverb or adverb phrase that comments on the entire sentence.

Commenting adverbs include such terms as fortunately, surely, perhaps, of course, and so on. Since readers usually hear a mental pause after these terms, they expect a comma.

Happily, our investigation turned up no surprises.

To be sure, some researchers offer conclusions that contradict this claim.

3. Put a comma after long introductory phrases or clauses.

Although the Japanese have proved to be world leaders in industrial design, especially in consumer goods, they have largely failed to export goods that reflect that leadership.

Despite the many concerns about the safety of nuclear power plants and their waste disposal, it seems inevitable that the United States will increase its nuclear footprint.

This is not a hard-and-fast rule. You have to exercise judgment in deciding when an introductory phrase is long enough to need a comma. Consider adding a comma when an introducer reaches ten words. But two other factors are relevant. Readers are less likely to expect a comma when two clauses have the same grammatical subject and are close in meaning. Compare

Once the IRS accepted our supporting documentation it agreed to allow the deductions for both the home office and the computer.

Although the IRS has few auditors and antiquated data processing, taxpayers remain fearful enough to be deterred from cheating.

You do not need a comma for short introductory phrases or clauses.

In a 2008 survey the CDC discovered fear of AIDS had become dangerously low.

To the untrained eye one cornfield looks like every other one.

If the inflow is reduced the pressure will remain within system tolerances.

4. Put a comma after an introductory element if it might confuse readers about the structure of the sentence.

NOT: When the speaker concludes her presentation will be all that most of the audience knows about her.

BUT: When the speaker concludes, her presentation will be all that most of the audience knows about her.