A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007
Dash
Spelling
Part III. Style 20 spelling
A dash is an elongated hyphen used to set off text in a way similar to but more prominent than commas (see 21.2) or parentheses (21.8.1). Technically called an em dash (before the introduction of computerized fonts, it was exactly the width of the capital letter m), this character is available in most word processors.1 It can be represented with two consecutive hyphens, but most word processors can be set to convert double hyphens to em dashes automatically. Do not leave space on either side of the dash.
When you use dashes to set off a parenthetical element, pair them as you would commas or parentheses. Since they are rather intrusive, however, do not use more than one pair in any one sentence; instead, use parentheses for the second layer of parenthetical information.
The influence of three impressionists—Monet (1840—1926), Sisley (1839—99), and Degas (1834—1917)—is obvious in her work.
You can also use a single dash to set off an amplifying or explanatory element.
It was a revival of a most potent image—the revolutionary idea.
Use a dash or a pair of dashes enclosing a phrase to indicate a strong break in thought that also disrupts the sentence structure.
Rutherford—how could he have misinterpreted the evidence?
Some characters in Tom Jones are “flat”—if you do not object to this borrowing of E. M. Forster's somewhat discredited term—because they are caricatures of their names.
A dash may also introduce a summarizing subject after a list of several elements.
The statue of the discus thrower, the charioteer at Delphi, the poetry of Pindar—all represent the great ideal.