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.
Dear John, 6.

Between the chapter and verse in scriptural references.
John xix: 22.
7.