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“I Am the Gap Between Knowing and Saying”: The En Dash Responds to the Em Dash
(www.mcsweeneys.net)
Commented Bringhurst thus:
Use spaced en dashes – rather than close-set em dashes or spaced hyphens – to set off phrases.
[...] The em dash is the nineteenth-century standard, still prescribed in many editorial style books, but the em dash is too long for use with the best text faces. Like the oversized space between sentences, it belongs to the padded and corseted aesthetic of Victorian typography.
Used as a phrase marker – thus – the en dash is set with a normal word space either side.
(The Elements of Typographic Style, 2004, § 5.2.1)