Colons are one of those punctuation items that I often see misused by students -- if used at all -- because we tend to stay away from using those items with which we aren't certain.
When it comes to knowing when to use a colon, a little knowledge (as the saying goes) will go a long way.
First, I guess it's a good idea to know that the colon (like most items in English punctuation) comes to us from other languages. In this case, both Latin and Greek can lay claim to its origin. Therefore, the rules of usage are mostly agreed upon by the experts, among whom I certainly do not belong. :-)
Generally, that which comes after a colon is meant to clarify, simplify, prove, or explain in more detail, that which comes before it.
Read that previous sentence again. It's that important.
Now, depending on who the authority is; there are a few basic uses of the this particular punctuation item.
In a dialog without quotation marks, a colon is used after the name of each speaker.
The above is typical of play, or in screen writing when there is a lot of dialog to make it easier for the reader to understand and follow the dialog.
Another usage of the colon is when there is a statement followed by a list of items.
One of the more common uses of the colon is in the separation of chapter and verse of religious works and epic poems. It can also be used to introduce a quotation as in the example below.
Another use is to separate a title from a subtitle.John 3:16 is sometimes called the gospel in a nutshell. It says: "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life".
Sometimes, the when to use a colon is not always clear, however, it is often used when introducing a definition or the noting of a specific time.
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