I never really thought of it before, but do you know the prober use of n- and m-dashes, I'm not sure I fully understand it?
I do! And I’m so glad you came to me for an answer! I hope you’ll indulge me a full exploration.
The Chicago Manual of Style, which is a slightly-longer-than-1,000-page book on grammar and its proper usage, is one of the main guides that publishing houses use to ensure consistent usage across all their published works. It’s a behemoth of a book and sometimes difficult to understand, but it’s a great place to start for technicalities like this, and I still happen to have my copy of the most recent 16th edition from my time at a press.
The technical nitty-gritty:
The CMS defines em dashes and en dashes as the following:
em dash: A short typographical rule measuring the width of an em.
en dash: A short typographical rule measuring the width of an en.
Excellent. And completely unhelpful, thank you CMS.
Both of these definitions require understanding what an em and an en are, respectively, so let’s define those. Now, typefaces, or fonts, are measured in points, right–the default most folks go with is 12pt font. “Points” are how we measure the space each letter takes up within that font, and 1 point measures 1/72″ (0.01384 of an inch, if you want to be precise). Okay, that in mind:
em: A unit of type (font) measurement equal to the point size of the type (font) in question. If you’re typing in 12pt font, then your em is the length of 12 points.
en: A unit of type (font) measurement half the size of an em. If you’re typing in 12pt font and your em is 12 points, then your en is half that: 6 points.
Now, as units of measurement, em and en often help to define the length of various things in typed and printed materials. There are em spaces, en dashes, etc, and in the end, all it means is that one is double the length of the other. And then, of course, you’ve got hyphens which are even shorter than en dashes. So what’s the point? Why do we use them and when? (And this time I’m not talking about font size.)
When do we use hyphens?
MOST COMMON USAGE: compound modifiers/adjectives.
Hyphens are generally used to create compound modifiers. When we’re describing another thing such as a country that speaks English, it can be described as an English-speaking country. The reason it becomes a compound modifier and gets hyphenated is because when we take each part of the modifier and apply them separately to the thing they’re describing (English country, and speaking country), they are either incorrect, don’t make sense, or don’t accurately describe the sentiment behind the statement. They must be together to be accurate and correct, so they get hyphenated together (English-speaking).
Take one more example: The description “a dictionary-wielding professor” in a sentence would not be “a dictionary wielding professor” because the professor is neither “a dictionary professor” nor “a wielding professor.” They are instead a professor wielding a dictionary, therefore the compound modifier should be hyphenated.
The same concept applies with a phrasal descriptor such as “a never-before-seen magic.” It is magic, and it is also never-before-seen, but it’s neither a never magic nor a seen magic; the entire descriptor is “never-before-seen.” Or “a dog-eat-dog competition,” as the CMS provides.
Of course, there are a bunch of other reasons to use hyphens, too, including enhancing readability for some words whose prefixes make the word unwieldy, but compound modifiers are usually the most complicated for writers.
When do we use en dashes?
MOST COMMON USAGE: ranges such as dates, times, page numbers, etc.
Mostly, en dashes get used in place of “up to and including” for things like dates, times, and page numbers. Things like, “Read chapters 11–16,” uses an en dash, and also with number ranges where the range is unfinished, such as birth and death dates where the death is unknown or hasn’t occurred: Emma Bull (1954–).
The en dash can also be used when talking about institutions of higher education who have the same name, but different campuses: Minnesota State University–Moorhead and Minnesota State University–Mankato.
The CMS does specify some usages of the en dash with compound adjectives like hyphens, however it’s an editorial nicety and not something to worry about. It’s very technical when you can do this, so stick with hyphens and if an editor wants to change it, that’s their prerogative. In fact, the CMS states: “Ensuring proper use of the en dash is usually considered the manuscript editor’s responsibility; authors can generally avoid the en dash and use hyphens instead” (CMS 2.13, pg. 60). Hyphens are the way to go with compound adjectives.
When do we use em dashes?
MOST COMMON USAGE: off-setting/emphasizing information such as an aside or where parentheses might also be used || interruptions of thought and/or dialogue.
Em dashes are the most common of dashes (the hyphen not actually qualifying as a dash specifically). It gets used “to set off an amplifying or explanatory element and in that sense can function as an alternative to parentheses, commas, and colons—especially when an abrupt break in thought is called for” (CMS 6.82, pg. 333). The em dash is what’s used in pretty much everything, and probably any writing post I’ve ever written has one, given that I type much the way I speak. Anytime you put in two hyphens, Microsoft Word often corrects this to an em dash, and whenever I type two dashes in my posts, I mean an em dash, I’m just too lazy to do the Alt-code to actually put in the em dash.
Whether you choose to use em dashes or parentheses for your asides in your narratives is up to you and your personal style. I know that I flip flop back and forth depending on what feels right, but as long as you’re consistent within your own work, you’re fine with either one.
Additionally, we sometimes have sentence that’re phrased in a way where neither side is a complete sentence, and where a comma might be one way to punctuate it. A writer may choose to use an em dash to emphasize it instead: “Darkness, thunder, a sudden scream—nothing alarmed the child.” Sometimes names are also offset this way in sentences: “Alie—that was her name, wasn’t it?”
Sudden breaks in thought or purposeful breaks in sentences such as interruptions or a rephrasing of thought can use an em dash, too: “Will he—can he—recover?” “I don’t know! I just thought, maybe—” “Maybe? He might?”
Question marks and exclamation marks can precede an em dash, but commas, colons, and semicolons never do. Periods very rarely do. Instead, em dashes take the place of those commas, colons, semicolons, and periods.
Some European writing standards use em dashes in place of dialogue quotation marks, indicating new speakers at the beginning of the dialogue:
—Where are we going?
—South. That’s where the ports are, and we need a boat.
Other em dashes:
MOST COMMON USAGE: missing or omitted words || bibliographies.
There are also 2-em dashes and 3-em dashes.
2-em dashes are used in omitted words such as expletives, names, or missing words, word parts, or illegible words: “Lady R——, whose home on the banks of the Guileless —— was the most elegant of those at the time.”
The 3-em dash appears pretty much exclusively in bibliographies, representing the same author or editor named in the preceding entry, and ONLY for an author or editor.
So there you go, my curious friend. Mostly, you’re using em dashes.
If you’re curious, an en dash’s keyboard code is ALT-0150 and an em dash’s code is ALT-0151. What appears as a key on your keyboard is a hyphen. Most computers replace two hyphens with an em dash, and if it doesn’t it’s generally accepted typing slang for it.
Hopefully that’s clarified it a little bit, even inside all the technical definitions. Let me know if I can help with any more examples or if I can describe it in another way. Thanks for the question!