Alternative form of ( ) or — —. These are two sets of -- used to enclose parenthetical text, like two em dashes, especially when actual em dashes are not available.
2001 February 7, pieceoftheuniverse, “Re: Help please?”, in rec.humor.oracle.d[1] (Usenet; in English), message-ID <3a817466.2941062@spamkiller.sucknews.com>:
Speaking of Illustrator -- you weren't, but I'm pretending -- the way I'm going about comic art is on the computer.
