The colon marks the place of transition in a long sentence consisting of many members and involving a logical turn of the thought. Both the colon and semicolon are much less used now than formerly. The present tendency is toward short, simple, clear sentences, with consequent little punctuation, and that of the open style. Such sentences need little or no aid to tell their story.
Rules for the Use of the Colon
1. Before as, viz., that is, namely, etc., when these words introduce a series of particular terms in apposition with a general term.
The American flag has three colours: namely, red, white, and blue.
2. Between two members of a sentence when one or both are made up of two or more clauses divided by semicolons.
The Englishman was calm and self-possessed; his antagonist impulsive and self-confident: the Englishman was the product of a volunteer army of professional soldiers; his antagonist was the product of a drafted army of unwilling conscripts.
3. Before particular elements in a definite statement.
Bad: He asked what caused the accident?
Right: He asked, "What caused the accident?"
Napoleon said to his army at the battle of the Pyramids: "Soldiers, forty centuries are looking down upon you."
The duties of the superintendent are grouped under three heads: first, etc.
4. Before formal quotations.
Write a short essay on the following topic: "What is wrong with our industrial system?"
When the formal introduction is brief, a comma may be used.
St. Paul said, "Bear ye one another's burdens."
5. After the formal salutatory phrase at the opening of a letter.
My dear Sir:
When the letter is informal use a comma.
6. Between the chapter and verse in scriptural references.
John xix: 22.
7. Between the city of publication and the name of the publisher in literary references.
"The Practice of Typography." New York: Oswald Publishing Company.
The colon has been similarly employed in the imprints on the title pages of books.
New York: Harper & Brothers, 1880.
DeVinne remarks upon this use of the colon that it is traditional and cannot be explained.
The colon is sometimes used between the hours and minutes in indicating time, like: 11:42 a.m.
DeVinne does not approve of this, though other authorities give it as the rule. It is probably better to use the period in spite of its use as a decimal point, which use was probably the motive for seeking something else to use in writing time indications. In railroad printing the hour is often separated from the minutes by a simple space without any punctuation.