The semicolon (;) is to many writers a mysterious squiggle. Some avoid it entirely for fear of using it incorrectly, while others tentatively try it out without really understanding its purpose. Writers and editors talking about the semicolon use words like rhythmic, elegant, lyrical, and subtle. It’s powerful punctuation, to be sure, but what’s it for?
Some of the confusion about the semicolon arises from its form: Is it a kind of comma? A variation on the colon? Despite its appearance, it’s neither of these. Functionally, the semicolon is most closely related to the period. It creates a pause, but not a full stop, between independent clauses (complete sentences). Editor Merrill Perlman says using a semicolon is “like rolling through a stop sign, where you are aware of slowing, but are still moving forward.”
Use a semicolon to show a close connection between ideas:
A carbon tax can be revenue neutral. It can be offset by cuts in personal income taxes.
A carbon tax can be revenue neutral; it can be offset by cuts in personal income taxes.
The semicolon creates a smoother flow, and the repetition (can be) becomes rhythmic rather than wooden.
Unlike the comma, the semicolon isn’t followed by a conjunction (and, or, but, for, nor, yet, so).
A semicolon can emphasize a rhetorical pattern used to develop an argument or make a point; it can show cause and effect, comparison, contrast, or other patterns. I like to use a semicolon as the pivot point or fulcrum of the argument: there’s balance but also movement, tension and resolution, a turning point, and sometimes a surprise.
With increasing industrialization, carbon and other greenhouse gases have accumulated in the atmosphere; the earth is warming rapidly.
The externalized costs of air pollution often turn up in increased health-care expenditures; those of climate change may be seen in billions spent on disaster recovery.
Most of us are aware of the environmental benefits of energy-efficient buildings; less well known are their social benefits.
Use transitional phrases if needed to make the pattern clear:
Relatively few highways in Canada require tolls; as a result, commuters have little incentive to take regional transit.
Changes in climate will shift agricultural zones northward; similarly, animals may migrate northward or to higher elevations.
There are limits to what can be achieved through regulation; carbon pricing, however, is the surest means of reducing emissions.
In a long complex sentence bursting with subordinate clauses and prepositional phrases, a semicolon can help the reader catch a quick rest and reorient themselves before moving on. Essayist Lewis Thomas likens the semicolon to a bench part way up a steep path:
Although the science of anthropogenic climate change is well established and the predicted effects are beginning to be observed, there are those, who, usually for ideological reasons, do not accept its premises and dispute its findings; perhaps more than anything else, this relatively small but powerful faction has prevented real progress in cutting GHG emissions.
Semicolons are also used to separate list items that contain commas:
Carbon sequestration can happen through natural biological, chemical, or physical processes; agricultural practices that return carbon to the soil; and chemical technologies that capture carbon from the atmosphere and store it indefinitely.
Take note, however, of a caution from the style guide of The Economist regarding semicolons: “Don’t overdo them.”
The colon (:) is one of the trickier punctuation marks. You can think of the colon in a sentence as similar to an equal sign: what follows the colon defines, explains, or expands on what came before.
One flower stands above all others in poetry, prose, and legend: the rose.
Some seed-starting mixes contain coir pith: a spongy material made from the husk of coconut, which improves air porosity and drainage.
A colon can also introduce a list.
There are five taxonomic kingdoms: Protista, Fungi, Plantae, Animalia, and Monera.
But be careful—a colon is used before a list only when the introductory clause is a complete sentence on its own. A common error is using a colon in sentences such as the first one below.
✗ The five taxonomic kingdoms are: Protista, Fungi, Plantae, Animalia, and Monera.
✓ The five taxonomic kingdoms are Protista, Fungi, Plantae, Animalia, and Monera.
Other uses of the colon include separating a title and subtitle and introducing block-set quotations.