A manual for writers of research papers, theses, and dissertations, Ninth edition - Kate L. Turabian 2018
Hyphens and dashes
Punctuation
Style
21.7.1 Hyphens
Hyphens are used in a variety of contexts, including compound words (see 20.3) and inclusive numbers (23.2.4).
21.7.2 Dashes
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). Also called an em dash (because in most fonts it is approximately the width of the capital letter M), this character is available in all 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. But avoid using 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.
21.7.3 Multiple Dashes
When you quote from a mutilated or illegible text, indicate a missing word or missing letters with a 2-em dash (formed with two consecutive em dashes, or four hyphens). For a missing word, leave a space on either side of the dash; for missing letters, leave no space between the dash and the existing part of the word.
The vessel left the —— of July.
H——h? [Hirsch?]
The same technique can be used when you want to obscure a particular word.
It was a d—— shame.
A 3-em dash (formed with three consecutive em dashes, or six hyphens) is used in bibliographies and reference lists to represent the repeated name of an author or editor (see 16.2.2 and 18.2.1).