The Em dash.
On an apple it is shift, option and hyphen at the same time.
It is possible to write an em dash in a couple of ways on a pc. You can hit "space dash dash space", and wait for Autocorrect in MS Word to kick in. You can hit Ctrl-Alt-Number Pad Minus. You can hold down Alt and punch 0151 on the Number Pad, but it is ridiculously involved.
If you have a 'netbook' or laptop (with no number pad), the em dash is beyond the typist's reach at the moment, but if you find a way let me know.