Dashes are like flashing yellow lights. You have to slow down, but you’re not always sure how much to slow down or why.

• Know n from m.

The en dash, about the length of an n and a little longer than a hyphen, is used for a span of numbers or dates. To create an en dash on most computer keyboards, type the first number, hold down the Alt key while typing 0150, and then type the second number. (For laptops, hold down the Fn key with the Alt key while typing mjim, the equivalent to 0150 on a numeric keypad.)

1. John 11:1–54 tells about Lazarus raised from the dead.
2. Orson Welles (1915–1985) wrote War of the Worlds.
3. Texas has its hottest weather in July–August.

The em dash, about the length of an m and about twice as long as a hyphen, functions as an extra-long pause, just short of a complete stop. Modern writing tends to use it too much. Readers don’t like slowing down for no good reason. Opt for a comma or colon, or use a period and start a new sentence. Reserve your long pause for when they’re needed most. Always use an em dash to indicate interrupted speech. To create an em dash on most computer keyboards, type the first word, hold down the Alt key while typing 0151, and then type the second word. (For laptops, hold down the Fn key with the Alt key while typing mjij, the equivalent to 0151 on a numeric keypad.)

1. Terrence checked with his agent—William Howell—about agent, William Howell, about the status of his book.
2. “I want you to know I have—” Alex fell back, reeling from his wife’s slap across his cheek. (To make the ending quotes turn the right direction, you may need to type two double quotes and delete the first one.)
3. If you’re going out, I need three things—milk, things: milk, bread, and peanut butter.
4. “I’m thinking about—John, are you paying attention?” (A shift in thought is like interrupted speech. Use an em dash.)
5. With intricate detail, I described the Marauder J4—both its beauty and its importance. (A comma would have been okay, but the author wanted more emphasis on the ending phrase.)