Classic job interview advice: if asked to lunch, never order the spaghetti. Long, saucy pasta strands are almost impossible to eat with aplomb. Semicolons are the punctuation equivalent of a spaghetti dish.
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; however, there are three good reasons for using one. (See what I did there?)
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.
Seldom is a sentence structured this way. It’s used when a writer is trying to show that two ideas together say something more important than either alone. In the example of Ms. Green’s death, the first clause records the end of one life. The second clause puts her death into context with all the other deaths of WWI service members. Two different senses of finality, then, are united in one sentence.
Note: When writers accidentally substitute a comma for this rarely-used semicolon, it creates a comma splice. Semicolons are used sparingly, so they are not the first fix for a comma splice. Instead, fix a comma splice by either adding a conjunction after the comma or replacing the comma with a period. See “Most often used” (below) for these two fixes.
Occasionally used: If one or more items in a list contain a comma, then all the items are separated from each other with a semicolon. 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.
Many lists have one or more items without a comma. The rule holds, however: 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 a list whose items do not contain commas. Save the details for a separate sentence (or paragraph) on each of your list 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 an essay, you risk burdening your reader with long, heavy sentences. Readers respond better to writing composed of varied sentence lengths and rhythms.