Dashes can be used to set off material in the midst of a sentence. However, one should try to avoid using them at all. When choosing between dashes and other forms of punctuation (such as commas and parentheses), dashes should be used sparingly and primarily when the material deserves special emphasis. By contract, parenthetical ideas may be lost. In typing or word processing, a dash is formed by placing one space before and one space after two consecutive hyphens.
Incorrect错误使用: The judge knew the defendant——she had been his third wife——and he therefore recused himself.
Correct正确使用: The judge knew the defendant —— she had been his third wife —— and he therefore recused himself. (来源：英语麦当劳 http://www.EnglishCN.com)