Use spaced en dashes – rather than em dashes or hyphens – to set off phrases.
Standard computer keyboards and typewriters include only one dash, but a normal font of roman or italic type includes as least three. These are the hyphen and two sizes of long dash: the en dash – which is one en (half an em, M/2) in width – and the em dash—which is one em (two ens) wide.
In typescript, a double hyphen (- -) is often used for a long dash. Double hyphens in a typeset document are a sure sign that the type was set by a typist, not a typographer. A typographer will use an em dash, three-quarter em, or en dash, depending on the contest or personal style. The em dash in 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.