1. The en rule

[the dash which is as wide as the letter 'n']

is distinct (in print) from the hyphen and is used to join pairs or groups of words wherever movement or tension, rather than co-operation or unity, is felt: it is often equivalent to to or versus, e.g.

The 1914–18 war;
current–voltage characteristic
The London–Horsham–Brighton route
The Fischer–Spassky match
The Marxist–Trotskyite split.

• Note The Marxist-Leninist position; The Franco-Prussian war with hyphens.

It is also used for joint authors, e.g. The Lloyd–Jones hypothesis (two men), distinct from The Lloyd-Jones hypothesis (one man with double-barrelled name).

2. The em rule (the familiar dash)

[the dash which is as wide as the letter 'm']

is used to mark an interruption in the structure of a sentence. A pair of them can be used to enclose a parenthetic remark or to make the ending and resumption of a statement interrupted by an interlocutor; e.g.

He was not—you may disagree with me, Henry—much of an artist;
'I didn't—' 'Speak up, boy!' '—hear anything; I was just standing near by.'

It can be used informally to replace the colon (use 1).