You talked about separating independent clauses with semicolons, but when would you use a colon to separate independent clauses instead? Are the uses completely different, or does the author get a choice?
–Stephen
That’s an interesting question. When a semicolon joins two independent clauses, it’s basically saying, “Here are two equally important bits of information that are part of the same thought.” It’s being a little coy about the exact relationship between those clauses. Sometimes you can replace it with a comma and a conjunction, but in that case it’s up to the reader to infer what the conjunction would be. Sometimes, as in the final example I gave in my semicolon post—”Happiness isn’t something you experience; it’s something you remember”—there’s no way to replace the semicolon with any linking words while preserving the sense and meaning.
A colon between two independent clauses, on the other hand, can usually be replaced with something like “that is” or “because” with no real harm done. That’s because the colon, when used like this, is meant to explain, restate, or illustrate what came before, which means you usually have the option of turning it into a subordinate clause.
Here’s a couple of examples:
The insurance company declared the car totaled: Repairs would cost more than the car was worth.
The insurance company declared the car totaled, that is, repairs would cost more than the car was worth.
The dodo was utterly alone: It was the last of its kind.
The dodo was utterly alone because it was the last of its kind.
You can’t really use a semicolon in these sentences because you lose what the colon is signaling: that the second phrase is clarifying the first in some way. Two independent clauses linked by a semicolon could be related in a number of ways, but the colon is much more specific and therefore not usually interchangeable.
However! Every so often you might run into a sentence where either could work:
A famished Johnny tucked into the long pork eagerly: He assumed the phrase referred to some new breed of pig.
A famished Johnny tucked into the long pork eagerly; he assumed the phrase referred to some new breed of pig.
I think the colon is the best option here, but if this sentence came across my desk with the semicolon, I’m not sure I’d change it.
If you think either could work but you’re genuinely not sure which is best, you may be able to avoid the question altogether with a handy-dandy em dash.
Bonus advice: If the phrase after the colon could stand as a complete sentence on its own, many American sources recommend capitalizing the first letter after the colon. If the phrase that follows the colon is not an independent clause, however, the first letter should always be lowercase.
Ask Copy Curmudgeon 
Gah, looks like I’ve been using semicolons where I should have used colons!
In the example you give from the semicolon post could you replace the semicolon with something like “but rather”?
You could, grammatically speaking. I don’t recommend it in this particular case because I feel like it just hurts the flow of the sentence without improving anything, but it’s not wrong, and in other sentences a strategy like that would work fine.
I like the advice to avoid both semicolons and colons if you’re unsure and just use a dash instead. I often see sentences where a sentence-final phrase is set off with a semicolon when it really should be a dash or even just a comma.
I’ve seen that too. I wonder if part of the confusion comes from people who are taught that punctuation mostly indicates how long a pause to take if you’re reading aloud. In that case, a semicolon might seem like a compromise between a comma and a colon or period, which would explain why it gets used to set off a short phrase at the end of a sentence.
Concerning dashes, wouldn’t it be informal to use dashes over periods? Which one do style guides prefer — the dashes or the commas?
In the first sentence, I actually intended to write “commas” instead of “periods.” My bad.
Nah, dashes aren’t really informal—they show up less often in really formal writing, but they still show up.
As for style guides, well, they don’t dictate everything. This one is up to writers and editors to decide for themselves.
That’s a comforting thought. :D I kinda like them because they seem to add more drama than commas.
I rarely use a colon in this way when I’m writing, but I also rarely change/remove it when I’m editing. It helps sets the author’s tone, I think.
[…] equivalent of a typo, nor for the semicolon after “syntax,” though I would use a colon or an em dash—it’s not strictly wrong, so I’ll chalk it up to writer’s choice. […]