The good news
If you’re struggling with trying to decide between a colon or semicolon, you are not alone. A lot of confusion about this topic stems from this fact that in some cases you can use them interchangeably. Take a look at the following two sentences.
It was freezing that day: the thermometer read minus 4°C.
It was freezing that day; the thermometer read minus 4°C.(Please note that in American English, the first letter of a second clause would be in caps,
i.e. It was freezing that day: The thermometer read minus 4°C.
Both of them are correct, however, the nuance is slightly different. In the first, the statement following the colon is illustrative of the first clause, while in the second, the semicolon simply indicates that the two clauses are linked. In both cases, the punctuation mark in the middle of the sentence can be replaced by a full stop as both clauses can stand independently of each other as full sentences:
It was freezing that day. The thermometer read minus 4°C.
The fact that the difference is so subtle here effectively means that you won’t go far wrong using either, as long as both clauses are full sentences in their own right. But you shouldn’t take this as carte blanche for using colons and semicolons interchangeably. Take a look at the following from Charles Dickens’ A Tale of Two Cities:
It was the best of times; it was the worst of times.
It was the best of times: it was the worst of times.
The first is correct, while the second, wildly incorrect. That’s because the second clause, while connected to the first, is not illustrative of the first, but actually stands in opposition to it, telling us a paradoxical truth.
Colons and sentence fragments
As I mentioned before, colons are illustrative and, as such, it’s okay to tag a sentence fragment onto the end of them. Take the following sentence:
It was a sound he knew well: his mother’s voice.
A semicolon is not correct in this case as the second clause is not a complete sentence.
Both the colon or semicolon can be used in a list, but for completely different purposes. We always use colons over semicolons to preface a list.
Correct: He needed three things: a brick, a sock, and a rug.
Incorrect: He needed three things; a brick, a sock, and a rug.
While they never come at the start of a list, semicolons can sometimes have a place in lists. Take, for instance:
You should consider visiting the following places on your world tour: Bangkok, Vietnam, and Malaysia for the food; Hungary, Austria, and France for the culture; and finally, Tibet, India, and Burma for the people.
As you can see, we need the semicolon here to differentiate items on a list that is already packed with commas.
Colons can also be used before quotes, but this is a convention that is more common in film scripts than in fiction, where the comma in conjunction with speech marks is standard.
Tom: It’s early.
Tom said, “It’s early.”
While you may find yourself often using colons, whether it be for a list or to illustrate a point, semicolons shouldn’t be cropping up all that often in your text. In fact, some agents are not a fan of this punctuation mark at all. The argument against them is that if two clauses can work as independent sentences, then why have a semicolon at all? A few people even argue that excessive use of semicolons can come across as rather pretentious. So, when connecting independent clauses, my advice would be to use them sparingly if at all.
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