Last week I posted about en rules. An em rule (—) is twice the length of an en rule (–). They are also referred to as en dashes and em dashes.
You will often see em rules used closed up (no spaces on either side) as a parenthetical dash; this is usually the preferred style for US publishers (instead of spaced en rules).
A spaced em rule can indicate the omission of a word; a closed-up em rule can indicate the omission of part of a word. It can also be used closed up in written dialogue to indicate a sudden break or interruption:
'You probably shouldn't put your knife in the toast—'
It is unusual but you might see some writers use a single closed-up em rule to set off dialogue instead of using quotation marks:
—Will he make it for dinner tonight? —Not in this weather!
You may also see a closed-up em rule between an introductory noun (or nouns) and the pronoun introducing the main clause:
Cars, thunder, the neighbour's cat—nothing disturbed the plucky dog.
Em rules are used in indexes to indicate a repeated word, and they are sometimes used to indicate a repeated author’s name in consecutive bibliographic entries.