A colon introduces a list. For instance, “Lauren showed me her collection from the beach: shells, pebbles, and sea glass.” However, it's incorrect to use a colon if the list is a direct object or the object of a preposition. Here are two examples of lists that don't need colons: Lauren collected leaves, rocks, and feathers on her walk OR Lauren is interested in rocks, minerals, and fossils. Another mistake I often see when editing is using semicolons instead of colons to introduce a list.
A colon can also introduce a formal statement or quotation. For example, “The PTA released a statement yesterday: ‘The organization increased the scope of its teaching grants last year, and more teachers applied for grants for science and mathematics projects.’”
The second function of colons is to act as a separator. A colon separates two independent clauses when the second clause restates or explains the first. For example, “The store’s return policy is typical: it requires a receipt for cash back.” Often a semicolon can be used instead, particularly if the independent clauses are closely related.
Colons also act as separators between the following:
- Hours and minutes (7:05)
- Chapter and verse (Matthew 21:12)
- Volumes and number of volumes (Muse 18:8) or volumes and page numbers
- A salutation and the body of a business letter (Dear Sir:)