This may or may not be a real TeX/LaTeX question but rather a general question on punctuation:
How are the different dashes
-- "–" and
--- "—" supposed to be used?
The grammar school version (for English usage) is:
-(known as an hyphen) between the elements of compound words
--(known as an en-dash) for ranges "3--7"
---(known as a em-dash) punctuation for digressions in a sentence---though how it differs from a parenthetical comment I have never known---which is why you don't see it much
As Charles notes in the comments, you should probably consult whatever style guide you use (or are required to use) for a more comprehensive and detailed treatment. Especially for the tricky cases.
The answers and comments I have seen so far are incomplete and not entirely accurate in some regards. Rather than hiding all the information in comments, here is a summary. Note that I'm listing only the cases that occur most frequently and that seem to raise the most questions. I am not saying that my answer is exhaustive. (For example, I'm omitting discussion of the various uses of dashes in dialogues or quoted speech because (1) they pop up rarely in the (La)TeX world as they are mostly relevant to copyeditors of fiction (who can be assumed to have learned the rules otherwise) and (2) their usage is rare and thus not necessarily governed by hard-and-fast, rigid rules.)
The em-dash has very few recommended use cases nowadays.
(Note that usage of either em- or en-dashes for parenthetical purposes includes many cases where you visually see only one em/en-dash, simply because of the implicit orthographic rule that the beginning or end of a sentence "eats" one member of the "dash-dash parenthesis symbol pair".)
The en-dash has two frequent usages:
pre-–World␣War␣II(too bad Unicode en-dashes don't render correctly on this forum!) with
␣standing in for a fixed (non-stretchable) space (this question is about how to produce such fixed-width spaces), because semantically the compound has the structure [pre [World War II]], because the "pre" modifies the entire compound word/expression "World War II", not just the word "World". That is: Don't look at simple rules telling you about hyphenated adjectives, prefixes, etc. for nouns (these are too complex); instead pay attention to the semantic structure, because this is the only thing that ultimately counts. We want a separator that separates orthographic words (= clusters of contiguous letters) a tad more than an ordinary space while still indicating that things belong together. If people were writing "pre-World War II" directly (with a hyphen, which is normally (not clearly on this forum though) shorter than or of equal length as an ordinary space), the immediate visual parsing experience would be to understand this structurally as [[pre World] War II], which is semantically incorrect. Note that linguistically the fact that English orthography doesn't (unlike German orthography) treat all linguistic words as orthographic words ("swimming pool" is one linguistic word but two English orthographic words) is what essentially necessitates this usage of an en-dash, which is entirely absent from German orthography. For example, we write things like "Konrad-Adenauer-Stiftung"; here ordinary hyphens represent all levels of inner-compound connections. Even though the division between "Konrad" and "Adenauer" is tighter than the one between "Adenauer" and "Stiftung" (the semantic structure here is [[Konrad Adenauer]-Stiftung]), there is at least no illusion that "Adenauer" binds closer to "Stiftung" than to "Konrad" (which there would in English, if you used a space for the first gap and only a hyphen instead of an en-dash for the second gap). Note, the point here is that the en-dash has this effect, because it must be visually slightly wider than an ordinary space.
Q: Are there constraints against line-breaking around dashes?
(Now you might ask: Why would anyone want to forbid line breaks on either or both sides of a dash? I can think of two possible reasons: (1) Perhaps someone would want to treat all dashes like hyphens, by visual analogy. (2) One could think of the two hyphens in a parenthetical pair as an "opening hyphen" and a "closing hyphen" and by analogy with other parenthesis types forbid the former line-finally and the latter line-initially. But such a distinction is never made in practice.)
Wikipedia claims en-dash usage for "relationships and connections", but Chicago does not recognize that; this "relationships and connections" usage is IMO newfangled and not generally accepted. But then, note as a disclaimer that I don't agree with all of Chicago. (Because many of its statements are prescriptivist recommendations, but I am trying to be descriptivist. Prescriptivism can be good, but many recommendations I've read - here and elsewhere - are neither descriptively accurate nor backed up by argument.)
Some thoughts and observations about page ranges:
pp.~100--200), but note that such usage is by no means universal even in the US. Subjectively speaking, I think it is used more in English than in German, though. With this in mind, German usage could be changing, and I also wouldn't be surprised if technical subjects' literature is influenced by (La)TeX conventions. (The (La)TeX community tends to recommend an en-dash for page ranges.)
Note that Wikipedia offers some "prescriptivist poppycock" (that expression is taken from the Language Log blog, which I highly recommend): "The figure dash is used when a dash must be used within numbers (e.g. phone number 555‒0199). It does not indicate a range, for which the en dash is used [...]." It is actually quite unclear where these prescriptions come from. I definitely can say that such usage is not widespread, and I do not see an obvious reason: the hyphen doesn't have the same semantic status as a digit, neither in a digit string (where it acts as a separator) nor in a range (of page numbers, units in physics, ...). Semantically the only intuitive rule is that the separator should be wider than the digit separation (that is, wider than zero) and that it should ideally be narrower than other surrounding orthographic divisions, to make clear that the digit group is a close unit (on the phrase structure level). That is, it would be odd to use an em-dash in a page range (since a sentence containing that page range would then have the largest visual separator token be just that em-dash), but by the same reasoning an en-dash is also not appropriate. So, a hyphen seems entirely fine, and I am somewhat surprised that the oft-heard (in the (La)TeX community) statement that ranges necessitate an en-dash isn't supported by either solid references or a good linguistic argument that talks about tokens, orthography, and phrase structure. As for non-range digit groupings: dots and colons can also function as separating elements in a sequence of digits, but noone would ever think about wanting to set them in a monospaced font with equal-to-digit width.
Usage of the em dash, since that's what dmckee's explanation doesn't tackle: it's nicest to think of the em dash as a dramatic pause in a sentence, and it need not be used parenthetically. Chicago (6.87) calls it "the most commonly used and versatile of the dashes", and lists among its uses:
Don't overuse it! Em dashes stand out more than any other piece of normal punctuation and too many on a page can make the text look absurd. Use it only to add force to your writing, or to solve tricky to parse punctuation problems — and those are often best solved by finding a less complex way of making your point.
Bringhurst says (The Elements of Typographic Style, v3.2, pp80-81):
5.2.1 Use spaced en dashes - rather than close-set em dashes or spaced hyphens - to set off phrases.
5.2.2 Use close-set en dashes or three-to-em dashes between digits to indicate a range.
5.2.3 Use the em dash to introduce speakers in narrative dialogue.
In (La)TeX, (1) - gives you a hyphen, (2) -- an en dash, and (3) --- an em dash.
The previous comments are correct. But for the interested user, I found this article which describes the difference well---It also explains whether adding space before or after the 'dashes'.
Hope it helps someone.
09/15/02: En Dashes and Em Dashes
A number of you have written to ask us to explain the difference between the hyphen, the em dash, and the en dash.
Distinguishing among the Three
The hyphen is the shortest of the three and is used most commonly to combine words (compounds such as "well-being" and "advanced-level," for example) and to separate numbers that are not inclusive (phone numbers and Social Security numbers, for example). On typewriter and computer keyboards, the hyphen appears on the bottom half of the key located on the top row between the "0" and the equals mark (=).
In many instances, correct hyphenation can be a complicated issue. We have addressed it partly in an earlier tip (go to the tip archive on this Web site and find the tip on hyphenated adjectives), and we will discuss it in greater detail in a future tip. Today, however, our focus is on the two kinds of dashes.
Remember, though, that when using the hyphen, the en dash, or the em dash, you should put no space either before or after them. The only exception is with a hanging hyphen (see, for example, the word "nineteenth" in the phrase "nineteenth- and twentieth-century literature"). By definition, a hanging hyphen will have a space after it but not before it.
The em dash is the mark of punctuation most of us think of when we hear the term "dash" in regard to a sentence. It is significantly longer than the hyphen. We use the em dash to create a strong break in the structure of a sentence. Dashes can be used in pairs like parentheses—that is, to enclose a word, or a phrase, or a clause—or they can be used alone to detach one end of a sentence from the main body. Dashes are particularly useful in a sentence that is long and complex or in one that has a number of commas within it.
When we confuse the em dash with the hyphen, we make a sentence virtually impossible to read. Notice the sentence containing dashes in the preceding paragraph. If we had used a hyphen in place of each dash, it would seem as though we had hyphenated two pairs of words in the sentence: "parentheses-that" and "clause-or," neither pair of which makes any sense.
The en dash is slightly longer than the hyphen but not as long as the em dash. (It is, in fact, the width of a typesetter's letter "N," whereas the em dash is the width of the letter "M"—thus their names.) The en dash means, quite simply, "through." We use it most commonly to indicate inclusive dates and numbers: July 9–August 17; pp. 37–59.
Many people were not even aware of the distinction between the en dash and the em dash until the advent of word processors, when software programs enabled us to use marks of punctuation that once had been available only to professional printers.
Typing the En Dash and Em Dash
Our typewriter and computer keyboards lack individual keys that display either of the dashes. (The symbol above the hyphen is an underline, not a dash.) Before word processing, we had to type an em dash by typing two hyphens. Now, many word processing software programs will automatically turn those two hyphens into an em dash (if we correctly leave NO space before or after them).
We can also choose en and em dashes from a menu of symbols that do not appear on the keyboard. In Microsoft Word, for example, we can pull down the "Insert" window, click on "Symbol," and go to the "normal text" window. The en and em dashes appear on the bottom row.
In any software program that handles text, the em dash can be typed on an enhanced keyboard as Alt + (0151)—that is, hold down the "alternate" key and type, using the numerical pad on the right side of the keyboard, the numbers 0151. The en dash can be typed as Alt + (0150).
Although this information can be located (with effort) among the prior answers and their linked references, I'd like to make several points:
The em dash is also used in dialogue, to indicate that the speaker was interrupted, or otherwise abruptly stopped speaking. This is very useful:
"The other day, I---"
"Will you stop with your silly---"
"But I'm trying to tell you that---"
"Just shut up."
Distinguish that from the ellipsis
... which can either indicate omitted material, or (in dialogue) indicate that the speaker is trailing off, or has lost the train of thought.
Some typefaces, such as Adobe Garamond Pro, have a default em dash that is a full em wide, and has no bearings (space) at either end. Many users think that's overwhelming. So, there may be an alternative "shorter" em dash, and the user may choose to add just a tiny bit of space before and after the em dash. Adding tiny space is easy to do in TeX. Typing a space is wrong (too wide).
The difference between a parenthetical comment using parentheses, and one using em dashes, is probably a matter of style. The dashes are more prominent, and thus suggest a more prominent comment.
As noted, MS Word and other word processors substitute an em dash for two hyphens, whereas TeX uses three hyphens. I have seen references to the "demise of the en dash" in word-processed documents, since the user tends to either type one hyphen or two (obtaining an em dash), rather than choose a correct en dash from a menu.