A notion that a computer can be useless by day and useful by night. Example: slave away at your job programming databases in MS Access under NT by day, and work overtime programming a yet cleaner version of ncurses tetris under Linux by night. The beauty is that, while your computer is one and the same, it almost literaly has two personalities. The idea can be extended for multiple boot configurations: NT, Win9x, RedHat, Slackware, Debian,