The dashoften typed as two hyphens side by side with no space between the dash and the words on either side of itis used to connect groups of words to other groups. Generally, the dash does this in two ways: it separates words in the middle of a sentence from the rest of the sentence, or it leads to material at the end of a sentence.
1) Separating Words in the Middle of the Sentence
As described in our section on commas, writers often place a component in a sentence and set the component off with commas. Sometimes, however, you might wish to place special emphasis on the component, but commas are too weak to serve this purpose. If this is the case, you may wish to use dashes for added emphasis. For example, look at these two pairs of sentences:
All four examples are correct, but numbers 2 and 4 place more emphasis on the component within them because of the dashes. Also, you have probably noticed that number 4 is much clearer than number 3 because the dashes clearly mark where the component begins and ends, whereas the reader might become confused by all the commas in number 3. In other words, you can use the dash to make sure your reader clearly understands your point.
In addition, you have an added advantage when using dashes over commas: you can use a full sentence as a component. For example, examine these sentences:
Linda Simpsonher enemies call her the author of our nations economic woeshas resigned her office with the present administration.
Notice how economical your sentence is when you can interject another entire sentence into the middle of it. Combining sentences in this way accentuates the relationship between the ideas and helps you draw attention to the component within the dashes.
2) Adding Words to the End of a Sentence
You can also use a dash to attach material to the end of your sentence when there is a clear break in the continuity of the sentence. Here are two examples:
The president will be unable to win enough votes for another term of officeunless, of course, he can reduce unemployment and the deficit simultaneously.
These two samples show how you can attach added material to the end of your sentence.
Use dashes sparinglyonly for those occasions when you wish to show special emphasis. They can help you communicate effectively in certain situations, but you dont want to clutter your prose with too many of them.