Sorry to be so long, but I have been looking for a good comprehensive internet source on dashes; all the information seems to be in bits and pieces, and some of it is quite contradictory. Here is a good excerpt, however, from the Get it Write website:
'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.'
For other points, I shall fall back on my own opinions:
Basically, use a dash in a range when you haven't time or space or layout for the words, and don't be inconsistent. That's why 'from A-Z' looks bad. 'Lays between 2 and 4 eggs' must be used, because the dash means 'through', not 'and'. If you want to be clearer for a larger range, add 'inclusive': 'B grades for this test are 72%-87% inclusive' (or 'from 72% to 87% inclusive'!).
'Between 2 and 4 eggs' would mean 2, 3 or 4, merely because it would be silly for it to mean '3' when you could just write '3'. For the dates, which are open to more interpretations, you would have to specify the exceptions of course; but, for instance, various seminars could be held 'between October 8 and 11', meaning on the 8th, 9th, and 11th.
No, as Get it Write says, there is no space before or after, except in the case of the hanging hyphen. I personally, however, leave a space after m dashes because I like the look and the sense better.
Hope this helps.
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