What Semicolons Are For

An editor once excised the semicolons from my writing with the marginal note “Death to semicolons.” He changed every one of them to a period.

Not every editor is so vehement about this much-maligned mark, but those who are may be provoked by the fact that so many writers don’t know how to use it properly.

This lack of accuracy may come about because some people learn that a comma is a pause and a period is a stop. One could readily deduce that a semicolon is somewhere in between.

Almost, but not quite.

One place this does hold true is if you replace the comma in a run-on sentence with a semicolon. A run-on, you may recall from your last grammar class, is two independent clauses joined by a comma. It is a grammatical error:

Incorrect: She knitted a sweater, he wore it.
Correct: She knitted a sweater, and he wore it.
Also correct: She knitted a sweater; he wore it.
Also correct: She knitted a sweater. He wore it.

You can join your independent clause with a comma and a conjunction, a semicolon, or a period. Both the “and” version and the semicolon version convey the close relationship between the first clause and the second. If you read them aloud, they’ll have different rhythms. So I recommend trying it each way before you make a decision.
Where you cannot use a semicolon—though many people try to—is to join an independent clause to a dependent clause. The dependent clause cannot stand alone as an independent clause can. So it must be joined more closely than is possible with a semicolon.

Incorrect: She had to stop knitting; because her wrists ached.
Correct: She had to stop knitting, because her wrists ached.

The dependency marker because prevents the clause from standing alone.
Some adverbs, like however and therefore, are termed “transitional adverbs,” and when they join two independent clauses, they should be preceded by a semicolon. One example in Chicago:

The trumpet player developed a painful cold sore; therefore plans for a third show were scrapped.

Note that no comma follows the adverb therefore in this example. If your second independent clause is a very complex one, a comma may be wise. But it is not always necessary.
Another place you’ll need a semicolon is with expressions like that is, for example, and the like. Again, from Chicago:

Keesler managed to change the subject; that is, he introduced a tangential issue.

You may have learned to use semicolons in a series. Here’s where it gets tricky, kids. You don’t need semicolons in lists unless the items in the lists have commas in them. That prevents the whole series from running together. Chicago once more:

The defendant, in an attempt to mitigate his sentence, pleaded that he had recently, on doctor’s orders, gone off his medications; that his car—which, incidentally, he had won in the late 1970s on Let’s Make a Deal—had spontaneously caught on fire; and that he had not eaten for several days.

If the semicolon continues to perplex you, don’t worry. It can often be done without, as my semicolon-averse editor demonstrated. Or you can consult a copyeditor. Use the comment form to submit specific examples you’d like edited, or use the contact form if you’d rather not take your rough draft public.