Going through the internets, I’m not convinced they are different. Or if they are, I can’t see the difference!
I use em-dashes for parenthetical remarks in my thesis and papers, and en-dashes for date ranges. Neither with a space at the sides. I believe that this is generally the accepted usage for these, but I’ve also read (perhaps in the Elements of Typographic Style?) that en-dashes should now be preferred for parenthetical remarks. IMO, using an em-dash still looks better, though.
Is this a PhD thesis? If so, then you can ignore certain things your supervisor says (assuming, of course, that the layout of the thesis is not tightly defined in a booklet somewhere)! You have a large amount of latitude in writing your thesis, and his comments are only meant to be taken as advice.
No, it’s an M.A. Thesis. Haven’t decided if I want to do a PhD dissertation down the road. Only a dinky 125 pages or so (we music types don’t like heft, it seems), but when my first docket came back with more red than a baboon’s rear, that was a sign for me that I needed to slavishly heed his advice! 
I don’t know why anyone else objects, but I can tell you why I like the em-dash without spaces: it looks better to my eyes. With the spaces, the em-dash just takes up too much typographical real estate and it seems to interrupt the flow of the sentence more than without the spaces; it creates more of a break between the main clause and the parenthetical than is necessary. I’m not exactly sure why, but that’s the impression I get.
IMHO (and this point really is a matter of opinion because sources differ) I object to the spaces because you don’t need a break to separate a break. The em-dash is used instead of a hyphen to firmly draw the distinction you suggest. Surrounding the dash by spaces just seems to me overdoing it. However, that is the standard in many newspapers, such as the Washington Post which I read daily.
It looks like they’re joined. It also makes me feel like I’m chewing on tinfoil when I read it. Granted, that may just be because the style guide I’ve been using for the last 6 years insists on spaces around dashes, so that’s what I’m used to, but it just doesn’t look right.
Art historians use Turabian, too. And as I open up my trusty old A Manual for Writers of Term Papers, Theses, and Dissertations by Kate L. Turabian, my eyes fall upon the copyright page, where I read:
So much of Turabian is based on Chicago, though there may be a few differences. I know when it comes to bibliographic entries, Turabian follows Chicago.
As for the em-dash, Turabian says:
She apparently does not condone the en-dash. And since colleagues in my field* all follow Turabian, the en-dash with spaces around it looks wrong to me. But this, of course, depends on which style-guide your field uses. If music uses Turabian/Chicago, stick with it for your thesis.
*in the U.S., I should say.
When I had a book published by a London-based publisher, I discovered that the Brits (at least the English) use en-dashes with thin spaces on each side, while Americans use em-dashes, no space. On the other hand, I always use en-dashes for ranges, such as page ranges and I believe that’s standard. The system I use—Tex—always turns three dashes into an em-dash and two into an en-dash and spaces only if you leave them. Except in formulas, where a single dash turns into a minus sign and if you want another kind, you have to make it explicit.
There were other problems with the English publisher—they never heard of a subjunctive—but since I did the typesetting, I had the last say.
Hari, I don’t think that’s universally the case in the UK. The Oxford manual of style recommends em-dashes with no spacing.
Another exception is a comma in numbers: 100,000
This, I believe, is also true when a different symbol like . or raised . is used in place of the comma.
I think I saw that mentioned somewhere recently, myself.
Wikipedia is proving to be more helpful than I expected:
Am I wrong that the words em and en get their meanings from the letters? That an em dash is as wide as a lowercase m, and an en dash is the width of a lowercase n?
Personally, I like your reasoning here…possibly because in my own work this is exactly what I do (spaces on either side of the em dash).
I grant that one usually sees no spaces in literature, and spaces in newspapers, but logic tells one that words are separated by a space, and the fact that a longer pause is implied by an em dash seems no reason to suddenly discard this convention.
I have always found no spaces around the em dash inelegant and awkward…but maybe that’s just me.
I too use en dashes to indicate date ranges, sports scores, and breaks in telephone numbers.
In Maths, stuff like this is taken care of by LaTeX. Other stuff that style guides cover don’t actually matter, so different people have different style. I don’t know anyone who’s ever used a style guide. Russians, Brits, and us U.S. Americans all use commas differently, and no one gives a shit.
For bibliography entries, I either (1) google and copy, (2) copy from someone else who references it, or (3) copy and paste another one of my entries and change the content. I can’t be bothered to memorize the conventions.
There are two different threads on this subject and I forget what I’ve said where, so let me say this again.
You, the author, don’t ever have to worry about this stuff unless… you are told to use a particular style guide. This may happen if you are writing a formal paper, like a thesis. Some publishers also require the use of certain style guides. Newspapers would like their reporters to follow the paper’s conventions. Many professions have similar strictures. That’s why dozens of different style guides exist.
Mostly, though, style guides are for people who have to put the words into final print form.
Usage guides are helpful for authors, since they give formal advice as to what is acceptable in formal English.
Style guides and usage guides do overlap somewhat and this leads to confusion. However, as an author, the placement of en-dashes and em-dashes isn’t your concern. No matter what you do or what you prefer, the likelihood is that they will be changed to conform to the style of the print work. (This changes somewhat when you are self-publishing.)
The same for commas, BTW. You may not care, but editors, proofreaders, and copyeditors care a lot about them. Good. They are supposed to. It is part of their jobs. It isn’t part of your job as the author. You are getting the roles confused. (Self-publishing confuses the roles, too, which mostly means that self-published works don’t look as good for reasons that most people can’t put their fingers on.)
I just edited a book containing original work from 30 different authors. It was my job to go through and ensure that commas were used consistently, and that i.e. was written out exactly that way and followed by a comma and not a colon, and that e-mail and e-book had a hyphen, and a zillion other little things. The authors were responsible for writing. I was responsible for editing. Two different jobs; two different concerns.
I’m very much in favor of the em dashes, especially for old-timey effects in fiction like “It was in the year 17—— that I was directed to D——— Lane to enquire of a Mr. K———. What a d———d nuisance!” Nothing but an em dash will work for that purpose. It’s made with Alt 0151.
What vexes me is the distinction between the hyphen and the en dash. The difference between them is so minuscule that it’s hard to even see, so insisting on totally different functions for them is kind of hairsplitting. On top of that, the minus sign is supposed to be different from both of them? Sheesh. The plain hyphen on my keyboard was always good enough for all three. (But that’s hardly an argument, since typewriters way back when I learned to type lacked a numeral 1 and you had to substitute lowercase l. Even older typewriters, I’ve heard, had no zero and you had to substitute the letter O. Keyboards have advanced some since then.)
When I edited my law journal, our authorities, in order, were:
- The journal’s internal guidelines (very few rules, mostly filling gaps)
- The Bluebook
- The Chicago Manual of Style
As I remember it, hyphens are for hyphenated words and telephone numbers, en-dashes are for numerical ranges, and em-dashes are for parentheticals and such (one of our internal guideliens was lions, tigers, and bears; NOT lions, tigers and bears). There were no spaces on either side of the dashes. This all came out of the CMS. I, however, have always liked putting spaces around my em-dashes when writing for myself (my old law firm’s grammar guru insisted that however should never begin a sentence…).
It depends on whether you’re using a fixed-width (monospace) or proportional-width font.
In a fixed-width font, such as that used on typrewriters and older (non-GUI) computers, there isn’t really a distinction between a hyphen, an en-dash, and a minus sign. The same character can be used for all three purposes. And in my early computer days (when programming and such) I got used to hitting the “hyphen” key whenever I wanted a minus sign.
Nowadays, though, the most commonly used computer fonts are proportional-width, and it’s more complicated. In situations where the computer interprets formulas and does calculations (like in a spreadsheet), in my experience they want you to use the hyphen for a minus sign. But that doesn’t really look right—it’s too short—so that when I’m typing numbers and formulas for a human, rather than a computer, to read, I always use Alt-0150 to get an en-dash.
You are not wrong as to the origin (although it was the uppercase, not lowercase, IIRC). But that no longer applies in many (I’d even say “most”) of today’s fonts.
That’s one reason that I never use en dashes.