One of the most common errors I see when copyediting is confusion of em-dashes, en-dashes and hyphens. I don’t think these dashes are commonly taught in school—I know I learned the difference between them my first year in publishing. So here’s a quick lesson:

Em-dashes are the triple length dashes that separate ideas in a sentence.
Example: My dog—who loves to run—is chasing her tail.
Often em-dashes are used in place of parentheses.

En-dashes are (usually) double length dashes that are used to show ranges.
Example: The shrubs may grow 36–42 in. high.
En-dashes are usually used between numbers, like dates or measurements. In some fonts, en-dashes and hyphens are indistinguishable.

Hyphens are a single dash used within words (such as en-dash and em-dash), to join words (such as compound adjectives) or to break words at the end of a line.
Example: She wore an itsy-bitsy, teeny-weeny, yellow polka-dot bikini.

Like most grammar rules, these aren’t set in stone. Publications have their own styles. Many people use space–en-dash–space instead of an em-dash for example. Or they use a hyphen instead of a an en-dash. Or, when there is a hyphenated phrase containing a hyphenated word, the hyphens between words become en-dashes (as in the sentence before last).

To make it easy, here are three basics to remember:
1. Always use a single short dash (hyphen) to join words or break a line.
2. Always use a longer dash (en-dash or em-dash) to separate thoughts in a sentence.
3. Be consistent.

I suspect one reason people often use the wrong dash is that they don’t know how to insert the correct one. Hyphens are standard on any keyboard, including most smartphone keyboards. If you are using Microsoft Word, you can find both en- and em-dashes on the Symbol list on the Insert menu. On a Mac, the shortcut for en-dash is option+- and for em-dash is option+shift+-. If you can’t figure out how to make a real en-dash or em-dash, you can substitute a hyphen for an en-dash and two hyphens (–) for an em-dash.

Truth be told, these are old printer conventions, and as digital publishing evolves, things are going to change. As with most grammar quibbles, the important thing is to be consistent.