Because of their similar look and numerous usage rules, dashes can prove one of the most difficult aspects of punctuation even for experienced writers. Understanding a few guidelines for when and how to use each kind of dash can eliminate most usage problems.
Hyphens should be used in phrasal adjectives (also called compound modifiers) when they precede a noun (e.g., “They live in a roach-infested apartment” or “He is a right-handed hitter”) and in cases where two elements of a phrasal adjective precede the same word (e.g. “The study spanned several science- and math-related topics”). When a compound noun is part of a phrasal adjective, the compound noun should also be hyphenated, e.g, “I signed the case-drowned-prosecutor initiative to help the government get the funding it needs to try criminals.” Note that phrasal adjectives that follow verbs do not take on hyphens (e.g., “The hitter was right handed”).
The En Dash:
En dashes can be used in three situations: when specifying ranges, when contrasting values, and in place of hyphens in phrasal adjectives that have two or more elements that are open compounds. In the first case an en dash is used to show a range, as if substituted for the word “through,” e.g., “She visits New York September–November.” In the second case, an en dash is used to show contrasting values or a relationship between two nouns, e.g., “The score was 5–8” or “The Dodd–Frank Bill passed.” The third use for en dashes occurs with phrasal adjectives that contain two or more open compounds. Open compounds are compound words that are separated by a space, such as “real estate” and “World War II.” Instead of taking on a hyphen, phrasal adjectives with two or more of these words take on an en dash, e.g., “the pre–World War II years” and “World War II–era politics.” To make an en dash in Microsoft Word hold down Ctrl and the minus key (option and the minus key for Mac).
The Em Dash:
Em dashes can be used in a variety of situations with different meanings. Perhaps the most common is to explain an element in greater detail. A single em dash can function in this way, much like a colon, e.g., “Fear pricked my neck when I peered beneath the rock—maggots lined its crevices.” A pair of em dashes sets off an explanatory clause much like a pair of commas often do, e.g. “He alternately blabbered, gaped, and shuffled—his feet scraping across the ground as if bearing weights—while we, beside him, wondered what to do.” Em dashes are particularly useful in series (such as the example above), where adding a pair of commas instead of em dashes can render the sentence difficult to read. They can also be used either alone or in pairs to indicate sudden breaks or swerves in thought, e.g. “I wanted to go—didn’t I?—to the symphony” and “I was indeed wondering—anyway, I don’t like thinking about that.” An em dash can similarly be used to signify interrupted speech, e.g. “‘Frankly, I would love—’ ‘You never loved anything,’ he said.” To create an em dash in Microsoft Word use two hyphens, each connected to their surrounding words without spaces, and Word will automatically substitute an em dash.