An em dash is a
dash with a width of one
em. In
typewritten texts, the em dash is composed of two
hyphens with no surrounding spaces. The em dash is used to enclose and separate.
Enclosing dashes, like enclosing commas, dashes need partners. The dashes can be used as to replace enlcosing commas and add emphasis. Dashes also come in handy when commas might make a parenthetical appear as a serial list. For example, the sentence
"Your sister, Sarah, and I went wild last night."
looks like I engaged in a three-way with your sister and someone named Sarah, whereas
"Your sister--Sarah--and I went wild last night."
shows that I'm just letting you know which one of your sisters went wild with me last night.
Enclosing dashes should also be used
- for clarity when the enclosed element contains internal commas
- when the enclosed element is itself a complete sentence
- to emphasize the enclosed element.
The dash can also be used where one might otherwise see a colon--in an explanatory statement. These separating dashes are also used to spearate two clauses when the second amplifies or restates the first. A semicolon could also be used in this situation, but a dash adds further emphasis.
Please note that in this writeup, everywhere I use the unqualified term "dash," I really mean "em dash."