The garter tab cast-on method is a really clever way to start a triangular shawl. It allows you to set up your knitting in three perpendicular directions, without an obvious start or finish. The video below shows you exactly how to work the cast on, starting with a provisional crochet cast on, working through the garter tab and then picking up the stitches around the edges of the tab. This is the cast-on method I've used in my Bithynica shawl, shown above.