I've been fooling around with edgings other than garter stitch,on a triangular shawl, thinking that garter would not stretch as much as the stockinette-based fabric of the rest of the shawl.( Although sometimes perhaps you want this to happen so the thing stays on, but that is a different question...) I found that a 5 stitch edging of ribbing, e.g. PKPKP would have the same vertical stretch as the rest of the shawl, but not curl as a stockinette border would.. I started a shawl using this edge but haven't finished it yet, sorry. But so far, the edge is really nice.The ribbing kind of contracts like I-cord, but has the correct number of rows. And you can start it the way you do the usual 3 stitch garter-edged shawl, e.g.: provisionally cast on 5 sts, PKPKP, turn, KPKPK, etc, until you've done twice the rows needed for picking up on the edge, ( usually around 14 rows,) then pick up 7 sts along the edge of the strip you just knit, then undo the provisional cast on and pick up the 5 live sts. Now you'll have 17 sts on your needle, and you can start your shawl pattern. There is going to be a slight jog where the sts are 1/2 stitch off, but it isn't very noticeable and gets buried in the ribbing ( and is at the back of the neck anyway). Then you can start the pattern of the triangular shawl as usual, marking the 5 edge sts on both ends of the row as you would if doing a garter edge . See photos, and try it!