The semicolon has two functions in a sentence.

First Function

The semicolon’s main job is to connect two closely related sentences, creating a type of compound sentence:

Pirate Jack broiled tilapia fillets in butter; the scent summoned the crew to the ship’s mess for dinner.

Captain Diaz broached a magnum of champagne; it complemented the flavor of the tilapia

Full to the gills with food and wine, the crew pushed their chairs back from the table and sighed; they thanked their lucky stars they had signed on to this ship.

(Note: Subjects are bolded and verbs are italicized in all example sentences.)

• The relationship between these pairs of ideas is clear, creating ideal conditions for the use of a semicolon.

Examples of Unclear Usage

On the other hand, if the ideas in the two sentences aren’t innately, unmistakably related, a semicolon will only confuse and distract the reader:

Full to the gills with food and wine, the crew pushed their chairs back from the table and sighed; every morning, the captain’s parrot squawked to wake them up.

The reader, seeing the semicolon, believes the writer is pointing out some clear connection between the two ideas, but what could it be? It certainly is not clear; in fact, the two ideas are jarring in their discontinuity.

To take the matter further, even if the ideas are similar, if their connection is not obvious, the semicolon is still not a valid option:

Captain Diaz broached a magnum of champagne; she kept a full wine cellar.

In this case, the reader is still likely to get side-tracked thinking about how having a full wine cellar follows from Captain Diaz opening champagne, leading to distraction from the flow of ideas.

In fact, it’s better to abstain from semicolons entirely rather than to use them in questionable circumstances. It’s also better to abstain from semicolons rather than use them too often. The semicolon should be saved for those rare, perfect times when the writer needs to insist that the reader connect two ideas that are so closely related that the reader needs no hints as to how they are related.

Second Function

The second job of the semicolon is to separate items in a series that already contains commas (sometimes called internal punctuation):

The pirate ship, Dophin’s Revenge, called at Port-au-Prince, Haiti; Nassau, Bahamas; and Kingston, Jamaica.

The officers of Dophin’s Revenge were Arabela Diaz, Captain; Cedric Cheboi, first mate; and “Pirate Jack” Manninen, cook.

All of the dastardly pirates had signed on for the same reasons: to sail the seven seas, especially the seas that were more fun than others; to drink a significant, yet finite, amount of grog; and, more important than anything else, to get rich, partially through stealing treasure chests, but also through clever insider trading and underhanded business practices.

If items like these are separated by commas only, it can be unclear where one item ends and the next begins, or even how many items are being listed. Readers may be unable to sort out the information, and even if they do, the transmission of ideas will be slowed down.