The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 5, 'Punctuation'
Printers use both a short and a long dash.
The short dash (‘en rule’) is used to indicate a span or a differentiation and may be considered as a substitute for ‘and’ or ‘to’ (but see 8.1):
the England–France match; the 1939–45 war; 2 January–13 February; pp. 81–101
However, compound adjectives take a hyphen and not a dash; thus ‘Sino-Soviet relations’ but ‘the Molotov–Ribbentrop Pact’.
Long dashes (‘em rules’), often with a space on either side, are normally found in pairs to enclose parenthetical statements, or singly to denote a break in the sentence:
Some people — an ever increasing number — deplore this.
Family and fortune, health and happiness — all were gone.
Long dashes should be used sparingly; commas, colons, or parentheses are often more appropriate. Other punctuation marks should not normally be used before or after a dash.
A very long dash (——), known as a 2-em dash, is used to indicate ‘ditto’ in bibliographies and similar lists:
Marlowe, Christopher, Edward II
—— The Jew of Malta
For means of representing the different dashes in typescript, see 1.3.5.