A certain unnamed colleague (you know who you are!) has a habit of using semicolons where he should in fact be using colons. So I thought a brief primer might be helpful.
There are two main instances in which semicolons are used. The first is to separate two independent clauses — each of which could be a complete sentence — that are somehow linked. In such a case, the semicolon could in fact be replaced by a period or by and. But the semicolon is nicer because it implies that the second clause clarifies or expands upon the first. For example:
Bob inserted a semicolon into the sentence; for once he got it right.
Note that the clauses both before and after the semicolon are complete sentences. A period would work, as would and, but a semicolon is more elegant. It implies that something related is coming, often a twist of some sort.
The second useful and correct application of the semicolon is to replace a comma, where necessary to clarify which items are linked. This is usually done when you’re writing a list of things and “sub-things”, each separated by commas. For example:
VKI Studios has clients in San Francisco, California; Detroit, Michigan; and Paducah, Kentucky.
Note that if you replaced the above semicolons with commas, you’d have a confused mess because the states would be all mixed up with the cities.
A colon is used to introduce something. It implies, “and here it is…” Often, the colon introduces a list. For example:
The American flag has three colors: red, white and blue.
Note that the portion of the above sentence following the colon does not make up a complete sentence. A semicolon would therefore not be appropriate. (Nor would and, obviously.) A colon is nicer anyway, as it implies “and here they are”.
Sometimes a colon isn’t introducing a list, but rather answers a question or completes a thought. For example:
I have just one question: why are we here?
Note that in this particular example, the section after the colon could in fact make up a complete sentence. But the glaring “and here it is” demands a colon.
That’s about it, really. Yes there are other applications for colons and semicolons. And there are instances where other punctuation marks (most notably, dashes) would be preferable to either. Feel free to chime in with examples and exceptions. But hopefully some readers will find the above helpful.
Are you reading, “Bob”?