Dashes are a form of punctuation that is pretty much guaranteed to show up on both the ACT® English Test and the multiple-choice SAT® Writing Test. Because they tend to be used more frequently in British than in American English, they are typically the least familiar type of punctuation for many students. That said, they are relatively straightforward.
Dashes are tested in three ways. The first is extremely common, the second less common, and the third rare.
1) To set off a non-essential clause (2 Dashes = 2 Commas)
In this case, dashes are used exactly like commas to indicate non-essential information that can be removed without affecting the basic meaning of a sentence. If you have one dash, you need the other dash. It cannot be omitted or replaced by a comma or by any other punctuation mark. This is the most important rule regarding dashes that you need to know.
Incorrect: John Locke–whose writings strongly influenced The Declaration of Independence, was one of the most important thinkers of the eighteenth century. (more…)