I would estimate that 10% of all editorial corrections I make at the level of copyediting are connected to the use of hyphens, en dashes, and em dashes. “Though the differences can sometimes be subtle—especially in the case of an en dash versus a hyphen—correct use of the different types is a sign of editorial precision and care.” (cf. Chicago Manual of Style, 17th edn. [CMOS 17] § 6.75)
In the majority of cases:
- Hyphens are used in compound words and as a separator (e.g., in a phone number). Note, however, that not all compounds are hyphenated: e.g. common denominator, birthrate. (cf. CMOS 17 § 7.82) If you have the final say, do not use a hyphen to indicate a page range. Depressingly and inexplicably, many publishers’ house styles specify the use of the hyphen to indicate a page range. This
- En dashes are used for ranges. A good rule of thumb is that unless it stands for a preposition like “to,” “through,” or “until,” the en dash should not be used. Conversely, it should always be used in such cases, and neither the hyphen nor the em dash should be used. The en dash can also substitute for a minus sign, although the latter is actually a different Unicode character.
- Paired em dashes are used—without spaces before or after—for abrupt punctuation. Although some writers freely use them in the place of commas, parentheses, or colons, it is good style to reserve their use for abrupt breaks, or when the use of other punctuation would be unclear. The overuse of em dashes obscures syntax. (The use of the paired en dashes, with spaces before and after each, is British or German. American usage and only uses the unspaced em dash for punctuation. Do not mix the styles!)
- A single unspaced em dash joins nouns, phrases, or even clauses in apposition, or where one glosses the other; it has the sense of “namely,” or “that is.” It never connects different parts of speech, much less disparate clauses. A single unspaced em dash may also set off that attribution of a quotation that is not itself set off by quotation marks.
- The double em dash is used to suppress expletives or names in running text. (As in, “What the f——, this f—— style guide requires that I use spaced paired em dashes for parentheticals and hyphens for page ranges. What a bunch of f—— loons.”) The triple em dash is used to elide the names of authors in a bibliography.
To summarize: the hyphen is always and only in compounds or as a separator, the en dash usually is an abbreviation for a preposition, and the em dash is usually a punctuation sign.
Hyphenation with “century”:
This is a perennial bugaboo, even among accomplished writers.
- twentieth century
- twenty-first century
- twenty-first-century (not “twenty-first century”)
- early twentieth-century (not “early-twentieth-century,” cf. § 7.87)
- mid-twentieth-century (cf. § 7.89)
Cf. § 7.88 for further examples with other common words.
Sometimes, an en dash can be used in the place of a hyphen to facilitate the parsing of long compounds (CMOS 17 § 6.80):
- The antique-organ–versus–neo-
baroque-organ situation is interesting. “Antique-organ” and “neo-baroque-organ” are two compound adjectives joined by hyphens, which are in turn compounded into one massive compound adjective joined by en dashes. The two glyphs encode the two hierarchical levels in the structure of the compound. Such “heavy” compound adjectives can be unintentionally humorous or turgid in style, so use with caution!
- He’s a no-fooling-around–nothing-but-business–get-things-done kind of guy. Here, the turgidity and humor come together an an effective way, because of the context.
- Twenty-first-century politics is complicated. Sometimes, a distinction is intelligible (“twenty–first-century” is logically not equivalent to “twenty-first–century”), but there is no risk of confusion. In such cases, just use hyphens.
- Snafu is actually an acronym meaning situation-normal–all-f––d-up. While there are two syntactic units (or fragments) in this phrase, it’s not really necessary to use the en dash between normal and all because no confusion is likely to arise. All hyphens could suffice, though some editors might prefer the en dash to aid visual scanning. However, because of the doubled em dashes for the expletive, and because it is not actually a single compound noun but a phrase, it would be better to set the acronym in quotations as:
- Snafu is actually an acronym: “Situation normal: all f––d up.” Either of the colons could theoretically be replaced by a single em dash, since they connect nouns in apposition, but again the use of the doubled em dashes for the expletive suggest that the colon is clearest.
- A situation-normal–all-f––d-up is likely when using punctuation promiscuously. Here, the expression truly functions as a grammatical compound noun (not a phrase), and so setting it with hyphens (and the one optional en dash, if desired) would be appropriate—yet note the unpleasantness of the style.