In my editing, I increasingly see the use of dashes, or the use of hyphens instead, used improperly. There are two kinds of dashes: the en-dash and the em-dash. The following definition for an en-dash comes from The Chicago Guide to Grammar, Usage, and Punctuation by Bryan Garner: An en-dash is a horizontal line that marks a span, a tension, or a pairing of equals. Examples are: good–bad dimension or a London–Paris flight (but do not use it to replace “from-to” phrases: “I flew from London to Paris”.
In academic writing, the en-dash should be used in number sequences: p. 200–201; 1972–1983. In essence, the en-dash translates to “to”: page 200 to page 201. A hyphen is often used instead—I think—because the dash is not on the keyboard.The shortcut for an en-dash on a PC is the following: the “ALT” key plus 0150. Hold the key down while typing the numbers and then release it. The dash will appear. When using this dash there should be no spaces around it unless the journal requires it.
The em-dash is more complex. Again, referring to Garner’s book, the definition of an em-dash is as follows: An em-dash is a horizontal line that marks an emphatic insertion, an informal introduction, or a sharp break in thought. Examples are: “In America—as elsewhere—free speech is confined to the dead” (Mark Twain). Here it sets off a phrase modifying America. “They say—the astrologers, I mean—that it will get better and better for me as I go on” (Henry Miller). In this case the dash is being used instead of parentheses to highlight the phrase. The first two examples come from fictional works to add nuance and probably are not appropriate for formal writing. Another use is as a device to hesitation in the sentence.
The most appropriate use in academic writing would be to set off a list in the middle of a sentence: “This market has everything—meat, fish, vegetables, and dairy—that the supermarket has. In this usage, the dash is better than using a colon. However, a sentence should only have two em-dashes at most. Also, they stand alone with no punctuation before or after and, like the en-dash, no spaces. On a PC the shortcut for the em-dash is “ALT” plus 0151. I recommend using the em-dash sparingly in formal writing. It is better suited for fiction.