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There are three types of "line thingies" you can use for various purposes; they are the hyphen, the en dash, and the M dash. Visually, the difference is that a hyphen is short (long enough that it isn't confused with a period or comma), the en dash is longer (by convention, the width of the capital N), and the em dash is the longest (the width of the capital M). A brief example of the most important types of uses for each follows.

I was taught that a hyphen was used in text for hyphenated words or to indicate a syllable break at the end of a line. It can also be used as a "minus" sign in mathematical equations. In American phone numbers, it's always used between the last and next-to-last part, and often between the area code and local part. It can also be used when you are using a phrase as an adjective (as in "next-to-last" above). If you are an Orthodox Jew, you use one in place of the O in "G-d." The proofreaders' mark for a hyphen is a = sign.

To type an en dash, you must substitute a hyphen (usually without spaces around it), or else use a special code to insert one in your word processing program. It's used to indicate a range, such as A-Z or 5000-6000. It can also be used in the very rare case of a hyphenated compound made from hyphenated words. The proofreaders' mark is like a fraction with a "1" over an "n".

An em dash is best typed as two hyphens--you might sometimes also see a hyphen with spaces around it. It's used in some of the same ways as a semicolon--to show pauses and set off parenthetical comments--but it has many of the same problems as a semi, too, as you can easily create huge run-on sentences if you're not careful. It's sometimes used if you don't want to use swear words or full names of people, like so: "Ms. S-- hit her thumb with the hammer and said 'D-- it!'" The proofreaders' mark for the em dash is like a fraction with "1" over "m".

I gave up proofreading when I stopped getting paid for it... lol.

In the first paragraph above (it SHOULD be above, but the posts show in REVERSE chronological order... this is a hassle), it should be "the en dash, and the em dash." Calling them N dash and M dash is not, strictly speaking, wrong, but it's best to be consistent.