1. When the members of a compound sentence are complex in construction or contain commas: e.g., He was courteous, not cringing, to superiors; affable, but not familiar, to equals; and kind, but not condescending, to inferiors.
2. To connect successive sentences: e.g., His face never showed an emotion other than that which he wished to have seen there; the mouth was protected by his heavy mustache; his eyes penetrated the object on which they fixed themselves. Shorter sentences should be divided by commas.
3. Between expressions in a series which have a common dependence upon words at the beginning or end of a sentence: e.g., The half-sick man is a nuisance to his entire household: he is not ill enough to accept restraint; he is too ill to be reasonable.
4. To separate passages containing chapters in scriptural references: e.g., Matt. 1 : 4–8, 12, 16; chap. 3; 8 : 16. {10}
THE COLON
The colon is used between clauses of compound sentences, additional clauses without a conjunction, formal quotations, and particulars in apposition with a general term, to mark the necessity of a pause greater than that indicated by the semicolon and less than that indicated by the period. The colon is much less used today than formerly, as modern writers employ shorter and less formal sentences than those of a century ago. It should always be placed outside the quotation-marks unless included as a part of the quotation itself.
Use the colon:
1. Before as, viz., that is, namely, etc., when these words introduce a series of simple particular terms in apposition with a general term: e.g., There are three genders in Latin: namely, masculine, feminine, and neuter.
2. Between two members of a sentence when one or both are made up of two or more clauses divided by semicolons: e.g., Vallie was nervous in his temperment; the Captain was calm and self-contained: Vallie was the product of the city; the Captain was the product of the world.
3. Between a complete sentence and an additional clause enlarging upon the same idea without a conjunction: e.g., Imagination {11} is essential to business success: without imagination, plans for future development cannot intelligently be made.