Hyphens form compound words. Hyphenated compounds can be nouns, as in “decision-maker” or “mother-in-law”; verbs, as in “double-check” or “freeze-dry”; or adjectives, as in “long-term” or “post-award.”

Hyphenated compound nouns and verbs typically have standard forms, found in the dictionary. Some compound adjectives likewise have standard forms, but they also get hyphenated in response to syntax, which helps readers group the words correctly on first read:

The late-stage prototype consisted of a modified commercial off-the-shelf (COTS) timekeeping device.

When several compound adjectives share a word, we can reduce repetition using a hanging hyphen:

short-, medium-, and long-term planning

Note that compound adjectives are typically not hyphenated under two conditions:

1. When one of the words is an adverb ending in -ly: e.g., “His poorly worded response went viral.”

2. When the compound adjective comes after the noun it modifies (often as a predicate adjective): e.g., “an eagle-eyed contracting officer” but “My supervisor is eagle eyed.”

En dashes are the middle child of dashes—the most elusive, the least understood. But that needn’t be the case, and a well-employed en dash will tickle the fancy of even the most punctilious punctuation connoisseur.

En dashes perform three main functions:

1. Indicating a range of values: e.g., a 1300–1400 appointment; on leave August 5–8; the years 1862–1867; volumes 2–4.

• This use of the en dash should not be preceded by “from” or “between”—e.g., not “on leave from August 5–8.” (To maintain parallelism, use the complete pairs “from . . . to” or “between . . . and.”)     

2. Indicating a connection or contest between discrete entities: e.g., the Tokyo–Osaka train; the Dragons–Wizards jai alai playoffs; the Iran–Iraq War; a 68–32 vote.

• Whereas a hyphen coveys unity, an en dash conveys separateness: “a French-Canadian conference” = a gathering of French Canadians; “a French–Canadian conference” = an event involving France and Canada.       
          
  • Note that, while this distinction can be useful in certain cases, some style guides, including the Chicago Manual of Style, use the en dash only to mean to (e.g., Tokyo to Osaka), not between (e.g., between the Dragons and Wizards); see CMOS 6.80.

3. Creating compound adjectives when at least one of the terms contains two or more words:   

• pre–Cold War (pre-Cold War = a war that occurred before things got cold)    

• a social media–centered campaign (a social media-centered campaign = a media-centered campaign with lots of social interaction)

The em dash is a versatile mark used to define, explain, and insert material. It thus performs many of the same functions as commas, parentheses, and colons, but with a bit more drama—a “wait for it” tension that tends to lend emphasis to whatever it introduces.

Em dashes can be used in the following ways:

1. Acting as a comma. Em dashes can stand in for several types of commas:

a. Framing an appositive or other nonrestrictive element:

• Under utilitarianism, an ethical theory that seeks to maximize well-being, the security guard’s actions were unjustifiable.    
• Under utilitarianism—an ethical theory that seeks to maximize well-being—the security guard’s actions were unjustifiable.
       
• The peace treaty, signed by all the belligerents, lasted for more than 400 years.
• The peace treaty—signed by all the belligerents—lasted for more than 400 years. 

• Grammar, logic, and rhetoric constitute the trivium, the core of a classical education.    
• Grammar, logic, and rhetoric constitute the trivium—the core of a classical education.

Note that dashes should not be used in this way if the information is restrictive.

b. Performing the function of a FANBOYS comma:

• You could use a comma to signal the next clause, or you could use an em dash.   

• You could use a comma to signal the next clause—or you could use an em dash.    

c. Standing in for a serial comma:

• . . . lions, tigers, and bears.  
• . . . lions, tigers—and bears.   

This usage would typically fall at the end of a sentence; placed in the middle, it would require another dash to "exit" the interposed information and return to our regularly scheduled syntax.

2. Disambiguating appositive lists. Here, the dashes replace nonrestrictive commas not for emphasis but for clarity:

Further talks took place among the member states, Romania, Peru, and Japan, resulting in a provisional agreement.

Are we talking about the member states plus three others, or are those the member states?

• Further talks took place among the member states—Romania, Peru, and Japan—resulting in a provisional agreement.
• Further talks took place among the member states, i.e., Romania, Peru, and Japan, resulting in a provisional agreement.  
• Further talks took place among the member states along with Romania, Peru, and Japan, resulting in a provisional agreement.    

3. Acting as a colon or semicolon. Similarly, dashes can perform the appositive function of the colon:

• Medieval students undertaking higher education learned the quadrivium: arithmetic, geometry, music, and astronomy.
• Medieval students undertaking higher education learned the quadrivium—arithmetic, geometry, music, and astronomy.

The appositive can also come up front, before the clause:

• Liberté, égalité, fraternité: this was the credo of the new republic.

• Liberté, égalité, fraternité—this was the credo of the new republic.     

Be aware that this construction creates something of a high-flown rhetorical effect; use it judiciously!

Em dashes can even join independent clauses like a colon or semicolon—just be aware that this usage is often considered less formal: 

• The Athenians then retreated from Syracuse; however, during this anabasis, they suffered heavy losses.
• The Athenians then retreated from Syracuse—however, during this anabasis, they suffered heavy losses.    

4. Acting as parentheses. Like parentheses, em dashes are able to introduce clarifying information:

• Under utilitarianism (an ethical theory that seeks to maximize well-being), the security guard’s actions were unjustifiable.
• Under utilitarianism—an ethical theory that seeks to maximize well-being—the security guard’s actions were unjustifiable.

Also like parentheses, em dashes can even temporarily suspend the syntax of a sentence and be used to insert entire clawses—pardon, clauses—as well as asides or other discontinuities in thought:

• The bears were famished (it had been three days since they had last eaten), and they stared greedily at the river, searching for any sign of salmon.
• The bears were famished—it had been three days since they had last eaten—and they stared greedily at the river, searching for any sign of salmon.    

5. In some styles, the three-em dash is used in the bibliography or list of references when it contains multiple sources by the same author:

Macdonald, Helen. H Is for Hawk. New York: Grove Press, 2016.

———. Shaler’s Fish. New York: Atlantic Monthly Press, 2016.

———. Vesper Flights. New York: Grove Press, 2020.

It's advisable not to perform this substitution until the list has been finalized, as it interferes with automated alphabetizing (and creates more work if you forget the author!).