Dashes and Colons
When do you use a colon instead of a dash? Here is a simple explanation:
Use a colon after a grammatically complete lead-in sentence that formally announces a subsequent enumeration, explanation, illustration, or extended quotation.
1. The communication courses I am taking this term are as follows: Communication 110, Communication 130, and Communication 225.
2. The best approach works this way: decide on a topic early in the course, approach your instructor with the idea for assistance, draw up an outline with a working bibliography, and be prepared to rewrite at least twice.
3. If it wasn't for my computer I would forget everything: my appointments, my phone numbers, and even my own birthday.
Notice that before the colon in each case is a grammatically complete lead-in sentence, also known as an independent clause. ("The communication courses I am taking this term are as follows"; "The best approach works this way;" "If it wasn't for my computer I would forget everything." Each of these is a stand-alone sentence.) Hence a colon is a kind of introduction in the form of a complete sentence alerting the reader to the fact that a list is about to follow.
The use of a dash instead of a colon, then, can be explained as follows:
Use a dash when the word or word-group that follows it constitutes a summation, an amplification, or a reversal of what went before it.
1. Atwood, Hemmingway, Tyler, Helprin -- these are some of my favorite novelists.
2. If he sensed that his presentation was going poorly, he would become agitated, look anxiously about the room, speak too quickly -- responses that further undermined his performance.
3. I used to believe I was in perfect shape -- until that hike last Saturday convinced me otherwise.
Notice that a grammatically complete lead-in sentence does not necessarily precede the dash. What distinguishes the colon from the dash as a symbolic device is that the colon throws the reader's attention forward, whereas the dash as a linking device throws the reader's attention backward. What the colon signifies is that what follows is a specification of what was formally announced in the clause on the left-hand side of the colon.