A particularly common reason for using the colon is to introduce information related to a phrase preceding it. The use of the colon indicates that what follows is an explanation or an elaboration on what came before. The colon can be used to connect two parts of a sentence, with the second part describing a consequence of the first.
You may use a colon to connect two sentences where the second sentence recaps, refines, or explains the first. Using the colon to separate the two parts of a sentence is less common in scholarly writing than in informal writing and novels. A colon rather than a semicolon can be used between independent clauses when the second sentence is explaining, illustrating, paraphrasing, or expanding upon the first.
A semicolon keeps the two independent clauses separated a bit, as a period would, so that we can easily identify which ideas belong in which clause. A colon can be used to separate two independent clauses when a) the second clause is directly connected with the first (not merely loosely connected) and b) the emphasis is placed on the second clause. For example, one could use a colon after an independent clause to draw attention to a list, an appositive, or a quote, and one could use it between independent clauses if the second is summarizing or explaining the first.
Colons follow independent clauses (clauses that can stand on their own as sentences) and they can be used to introduce explanation, draw attention to something, or connect ideas. Colons are used at the end of an independent clause, but what follows a colon does not have to be the full idea in itself.
Often, the use of the colon is intended to separate a word or phrase in order to emphasize or call attention to it. While it might not be used as much as a period or a comma, a colon is helpful in connecting sentences and placing emphasis on a word or phrase.
The colon has various uses, some of which include initiating a list, initiating explanation, initiating quotations, and connecting two related sentences. The colon is in some sense attached to a claim, and is used to highlight a point, to extend or explain an idea, or to introduce a quote or a list. The colon can also be used to introduce a direct quotation, especially if the quote is long.
However, Chicagos Manual of Style requires that capitalization only occur when a colon introduces a direct quote, a direct question, or two or more full sentences. For instance, according to AP style, a person should capitalize following the colon only when that word is a proper noun, or that word begins a complete sentence.
Chicago Style agrees with the part about proper nouns, but it requires two full sentences following a colon, not just one. In American English, styles vary, but capitalize the first word after a colon is best, as long as the content follows it to make two or more complete sentences. If the full phrase follows the colon, as in the previous example, authorities are divided on whether the first word should be capitalized.
Capitalize the first word following the colon if the information following the colon requires two or more complete sentences. It should also be noted that when the colon is used to separate two clauses, the first word following the colon should not be capitalized, since the words following the colon do not constitute a new sentence. Use a colon when you are using a starting phrase which can stand on its own as a full sentence, with the last sentence or clause detailing the previous idea.
However, it is common to use a colon after a word, phrase, or sentence halfway through the text introducing some subsequent material which is inserted mid-page. When listing items one-by-one, one-by-one, following the colon, the use of caps and ending punctuation is not necessary when using individual words or phrases prefixed with letters, numbers, or bullet points. If a single word or fragment of a sentence follows a colon, using a lowercase is generally recommended, unless the colon is followed directly by a proper noun.
Avoid using a colon before a list if it follows a verb or a preposition that normally does not require punctuation in this sentence. In contemporary English usage, the full sentence will come before the colon, and the list, description, explanation, or definition will come after. A colon is usually not followed by a capital letter in British usage, although American usage frequently chooses to use one.
A colon may be used directly following the salutation in formal letters (less formal letters will usually use a comma at that position). The colon is a punctuation mark which, when placed after a statement, signals to the reader that more information is coming. Most often, the colon functions as a leading punctuation mark, notifying the reader that the coming information supports, explains, or expands on something said before the colon.
The colon may also be used prior to quotations in running text, particularly if the quotation is long; or when it is a formal statement, or if the statement is given particular emphasis; or when an entire independent clause precedes the colon. A colon may be used when an object following a colon extends or clarifies something that came before it. A colon introduces an element or a set of elements which illustrates or reinforces the information preceding the colon.
While you could also use a semicolon or period in-between two separate, yet related, clauses, a colon is slightly more flexible than a period, while it is slightly more rigid than a semicolon. Different styles of quotation marks (such as MLA, APA, Chicago, and AMA) have slightly different rules about whether you should capitalize the first letter following the colon.