The string "-- " (That's two dashes
and a space) on a line by its own, that goes between the message body
and the signature
posts. Helps the newsreader
strip out the signature in replies. Some also display the .sig
in a different colour.
Some pedantic people throw a wobbly if you don't include it or (heaven forbid) don't put in the space. Most newsreaders include this, but one or two (not mentioning any Microsoft products in particular) don't.