on Writing Forward:
The colon is one of the most clearly-defined punctuation marks. It occasionally acts as a stand-in for a comma or period (though when one of these other punctuation marks will do, the colon is unnecessary).
Most commonly, the colon functions as an introductory punctuation mark, notifying the reader that the forthcoming information supports, explains, or elaborates upon what has been said prior to the colon.
These punctuation marks are common in math and science as well as technical documentation. In creative writing, we don’t see a lot of colons unless we’re working on a script. Colons are most often seen in text where a list is being introduced.
Chris The Story Reading Ape's Blog 
Reblogged this on Kim's Musings.
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