Introduction

To help readers separate introductory material from the main part of a sentence, use a comma after a long phrase or subordinate clause that opens a sentence. Here are some guidelines to help you follow this principle.

1. Use a comma after a long introductory subordinate clause. A subordinate clause is a unit that contains a subject and verb and is introduced by a subordinating conjunction such as after, although, because, if, since, though, when, or where.

   Example: Although the United States once employed hundreds of lighthouse

   The main clause in this sentence is today there are only two. The rest of the material leading up to the main clause is an introductory subordinate clause and should be set off with a comma.

   Revised: Although the United States once employed hundreds of lighthouse keepers, today there are only two.

2. Use a comma after longer introductory adverbial phrases. An introductory phrase of more than five words should be set off from the main clause by a comma. Often these phrases will begin with a preposition such as after, at, between, for, in, over, through, or until.

   Example: Between the piers of the harbor two automated lighthouses guide ships during the night.

   The main clause of this sentence, two automated lighthouses guide ships during the night, is introduced by a long adverbial phrase. The introductory phrase needs to be set off by a comma.

   Revised: Between the piers of the harbor, two automated lighthouses guide ships during the night.

3. Shorter introductory phrases generally do not need to be set off by a comma.

   Example: On Thursday a ship ran aground in the harbor.

   However, a comma should be used if it will help to clarify meaning or prevent a misreading.

   Example: To many sailors seem to lead an exotic, independent life.

   Here it is easy to misread sailors as part of the introductory phrase. This misreading can be prevented by putting a comma after the introductory phrase.

   Revised: To many, sailors seem to lead an exotic, independent life.

Remember: Use a comma after a long introductory clause or phrase. Use a comma after a shorter introductory modifier if needed to prevent confusion.