Dashes are used to separate groups of words (My three brothers—Mike, Tom and Pat—live in different states), not to separate parts of words the way a hyphen does (merry-go-round).
According to William Strunk Jr. and E.B. White in “The Elements of Style,”
“A dash is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than parentheses.”
Two kinds of dashes exist, the em dash and the en dash. The em dash, like the letter “m,” is twice as long as the “en” dash. An em dash is created by typing a hyphen twice and then immediately typing (no space) a word. This omits the space between the two hyphens and creates a single horizontal line. An en dash is technically longer than a hyphen, but a hyphen is commonly used as an en dash.
Use an em dash:
To highlight words interrupting the middle of an independent clause. For example, I take my three favorite courses—algebra, biology and world history—before lunch. Or , Sorry, but I can’t meet you—I’m flying to Virginia in the morning—but I’d love to see you when I return.
To indicate interrupted speech in dialogue. For example,
“Hey, it’s my turn to decide.”
To emphasize a sentence. For example, I will bring the chocolate cookies to the shower—if only Bill would stop snacking on them!
To end a dateline in a news story. For example, Orlando—Hurricane Dorian stayed out to sea east of central Florida.
Use an en dash to show a range of numbers and to connect items of equal meaning. For example,
Please annotate pages 1–22 before our next class.
I work from 8 a.m.–5 p.m.
Notice the difference between the male–female brain size.