A critical eye can overlook a comma mistake or two without blinking, but a semicolon mistake might as well be red sauce on a white shirt front.
Semicolons are infrequently needed, which means people don’t get enough practice using them; however, there are three good reasons for using one.
Rarely used: Semicolons join two independent clauses without using a conjunction. It looks like this:
Florence Green, of King’s Lynn, England, died two weeks short of her 111th birthday; she was the last known surviving service member of WWI.
It is never grammatically necessary to structure a sentence this way. Instead, it is done to make two ideas more important than either alone. In the example of Ms. Green’s death, the first clause records the end of one life while the second places that one death in context with all the other deaths of WWI service members. Two different senses of finality united in one sentence.
Note: When writers substitute a comma for this semicolon, it is called a comma splice. Semicolons should be used sparingly, so if comma splices are often a problem, fix them by adding a conjunction after the comma or replacing the comma with a period.
Occasionally used: Semicolons separate all the items in a list if one or more of the items uses a comma. It looks like this:
The final three WWI service members were Frank Buckles, a U. S. Army ambulance driver in France; Claude Choules, a Royal Navy veteran and the war’s last known combatant; and Florence Green, who enlisted in the Women’s Royal Air Force just months before the war ended.
Not every item in the list has to have a comma. Semicolons are used if even one of the items has an internal comma. It looks like this:
The final three WWI service members were a U. S. Army ambulance driver named Frank Buckles; a Royal Navy veteran named Claude Choules, who was the war’s last known combatant; and Florence Green.
Note: If this is a rule you fear you won’t remember, then use short lists with commas and save the details for a separate sentence on each of your items:
The final three WWI service members were a U. S. Army ambulance driver named Frank Buckles, a Royal Navy veteran named Claude Choules, and Florence Green.
Most often used: Semicolons separate an independent clause from one that begins with a subordinating conjunction. It looks like this:
Marham Royal Air Force Base had planned to send a delegation to celebrate Ms. Green’s birthday; instead, they provided a bugler and a Union Jack for her funeral.
Note: You can avoid the semicolon if you break the sentence into two parts with a period:
Marham Royal Air Force Base had planned to send a delegation to celebrate Ms. Green’s birthday. Instead, they provided a bugler and a Union Jack for her funeral.
A final note: Be careful not to overuse semicolons. If you use more than one or two per page of essay, you risk burdening your reader with long, heavy sentences. Readers respond better to writing composed of varied sentence length and rhythm.