There are three kinds of dashes: em-dash (—), en-dash (–), and hyphen (-). Em-dash is used as an alternative to commas—as well as parenthesis. En-dash is commonly used to denote ranges of numbers, such as 1 – 4. The common hyphen is used to connect words in compound nouns, such as mother-in-law, and also to break longer words to the next line.

Depending on the software or tool used, it's not always easy to type the three different kinds of dashes. For instance, on Windows the em-dash may be typed by holding Alt and typing 0151 on the keypad—which is quite cumbersome. In such situations, it's common to type 2 – 3 normal hyphens to substitute the em-dash. 2 such hyphens then, would be a "double hyphen dash".
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