Em dash

Yes. Otherwise people would assume the em dash is a hyphen. I think it's a style issue though, and there are no hard and fast rules to it.

You will find that news articles and books are almost always written with no spaces. But as dariensan did say "there are no hard and fast rules to it", well, that may be true in everyday writing but when writing articles/essays/books/papers, the hard and fast rule is no spaces.

Typographers in the US use no spaces. Hard and fast rules. An en-dash is half an em-dash, and is used as a hyphen.

-
—

They are clearly different. What you will see occasionally is - used in place of an em-dash.

Guess I'm out of date. My last post stated the "typographers' rules" as I recalled them from the typography book _Words Into Type_. Examination of the web pages of the _New York Times_ and _The Washington Post_ show both using an ultra-long em-dash with what appears to be an en-space at either end. Other newspapers' web pages are using en-dash, with the spaces appearing to be full spaces.

There are, just to be contrary-- also situations where no space after or before is appropriate if not absolutely correct. In byline- dateline it is often the case that the byline receives no space but the em-dash takes one before the dateline.
In situations, rare though they be, where the author is stressing the moment of an event, the parenthetical phrase is preceded by a space, even when it follows an em instead of parens or commas at its beginning. If the initial thought is completed within the sentence then the parenthetical closes with a comma and the rest of the sentence progresses as if nothing had happened. The most frequent and annoying occurrance of the described usage-- is in fiction and, must be well constructed to avoid the hungry red pencil of an editor who reads it differently.

Coming in late, but I believe the US standard is to use no spaces on either side of an em dash, while the UK standard is to ALWAYS insert a small (kerned) space, usually an en-space, on either side of an em-dash.

erle is right for the most part. But mistaken about en dashes. They are not used as hyphens. Hyphens are used a hyphens. En-dashes are used for spans of time or measurements.

example:

20032004

The last two comments appear to be correct according to 2011 grammatical standards.