If you’ve ever doubted the colon’s usefulness, consider how it can transform the first sentence above: A woman: without her, man is nothing. If you’ve ever scoffed at the semicolon, look how it can alter the second: Twelve people knew the secret; all told.

Colons and semicolons may for some be little more than editorial esoterica, but for anyone who writes regularly, distinguishing between them and placing them properly is of the utmost importance.

Many imagine the colon and semicolon as close kin; however, the two marks differ considerably. As Sheridan Baker puts it in The Practical Stylist (5th ed., 1981), "A colon . . . signals the meaning to go ahead; a semicolon, as in this sentence, stops it. The colon is a green light; the semicolon is a stop sign." In more prosaic terms, the colon introduces what follows it; the semicolon does not.

Understanding the function of colons and semicolons is only half the battle; using them correctly is the other half. Arm yourself with the following pointers and you will emerge unscathed from even the roughest colon and semicolon skirmishes.

Using the colon

The colon normally introduces a list, formal quotation, summation or idea that somehow completes the introductory statement. The colon is a concise way of saying "and here it is," as in this sentence: "My mother gave me some excellent advice: to stop worrying about the things I cannot change."

The colon also says "and here they are" ("She has three hobbies: jogging, gardening and taxidermy") and sometimes "and here is why" ("Their lobbying efforts were ultimately useless: the bill was soundly defeated"). Think of the colon as the grammatical game-show hostess, gesturing at the prize awaiting you on the other side of the sentence.

The important colon rule—the rule most people forget, or indeed never knew—is that the mark should follow an independent clause, a group of words that can stand alone as a sentence. Here are some examples of correct colons:

• YES Her sister at last returned all the items she had borrowed: the broken leaf rake, the electric egg poacher, the string of orange Christmas lights and the portable karaoke machine.
• YES The scientists determined that only one outcome was possible: progressive genetic deterioration.

The most common errors result when writers place a colon not at the end of an independent clause but in the midst of one. In such cases, the colon often breaks an essential grammatical connection. Consider these examples:

• NO The movie would have been more enjoyable without: the buzzing in the speakers, the dust on the projector lens and the noisy popcorn eater in front of us. (colon separating preposition from its object)
• NO She believes without question, without a glimmer of doubt: that extraterrestrials will invade the earth sometime in the next decade. (colon separating verb from its object)
• NO The guest speaker’s address was: pompous, boring, self-congratulatory and entirely too long. (colon separating verb from its subject complement)

In each case, omitting the colon makes the sentence correct.

On another note, forget what your typing teacher taught you. A colon in a sentence is followed by one space, not two. In moving from the typewriter, with its non-proportional characters, to the word processor with its proportional fonts, we have lost the need for extra room after the colon.

Using the semicolon

Some writers relish the semicolon; others ridicule it. According to William Zinsser, author of the now-classic On Writing Well, "There is a 19th-century mustiness that hangs over the semicolon." That may well explain why the mark is such a favourite with modern novelist John Irving, a devotee of Victorian Charles Dickens (who was himself an extravagant scatterer of semicolons).

Fan or foe, you should bear in mind the semicolon’s valuable ability to connect related ideas in ways other marks cannot. It is particularly adept at joining contrasting ideas. This is a long-recognized use, with literary examples ranging from the profound—"A wit’s a feather, and a chief a rod; An honest man’s the noblest work of God" (Alexander Pope)—to the profane—"It’s not the men in my life that count; it’s the life in my men" (Mae West).

But semicolons are frequently misused, even by careful writers. The key is to remember that the semicolon links independent clauses. It should not join an independent clause to anything that is grammatically dependent.

• YES The auditors made nine recommendations; only one has been adopted.
• YES In the emotional aftermath of completing his tax return, he forgot to sign the document; however, he noticed the omission before sealing the envelope.
• NO Gaining and maintaining a high level of physical fitness takes a good deal of time; time that pays off in the long run. (one independent clause, one dependent)

In the last example, substituting a comma, a dash or indeed a colon would make the sentence correct.

Here is an easy reminder: use a semicolon only where you could also use a period. There’s only one main exception to this rule of thumb: when punctuating a list in which an item already contains a comma, use semicolons instead of commas to separate the items.

• YES Having recently sold his start-up company for a rumoured $9 million, Trey now spends his time relaxing in yoga and Pilates classes; experimenting with vegetarian recipes, especially Thai and Tibetan; and playing his electric flute for anyone who will listen.

The aristocrats of punctuation, colons and semicolons are not always at home in dressed-down, informal texts. But seat them correctly in your formal workplace writing, and they will add polish and distinction.

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