Many triangle shawls that are worked from the top down begin with a garter tab cast on. I know this may sound intimidating, but a garter tab is simply a small rectangular tab of garter stitch (usually 9 stitches), that blends seamlessly with the garter stitch edging along the top of the shawl.
In this video, I demonstrate how to start a shawl with a garter tab cast on. A garter tab cast on creates a small rectangular tab that blends seamlessly with the garter stitch edging of a top down triangular shawl.