|A shell stitch is formed by using your blind hem stitch on your sewing machine and using the regular presser foot (not a blind hem foot).
This technique isn’t new and was used a lot for hemming tricot lingerie and also used in heirloom sewing; I love using it with knits. Some machines have a shell stitch which is basically the same as a blind hem but mirrored, either will work you just need to position your material the direction so the V swings off the edge of the folded material when sewing. Check your manuals, too. My brother sewing machine uses a reverse blanket stitch that is on the machine to produce a beautiful shell stitch, it is explained very well with pictures in the manaual. You can experiment with different stitches, lengths, widths, and tensions using these stitches to produce the shell stitch look you want.
Example to try: take a 3 inch strip of material, tricot, interlock, or any soft material and using your blind hem stitch, shell stitch or mirrored blind hem (all work), position your material so the V of the stitch falls off the material edge and the straight part of this stitch is on the edge. Experiment with stitch width, I usually end up between 5-7 mm width swing. I tighten my upper tension to get the V to pull in a little more producing a 3-d shell. You will want to match your thread to the material to get the shell affect otherwise the stitches are too noticeable. Now just sew along the folded edge with the swing going off the material, and a shell is naturally formed. Play with width, length and tension until the effect you’re looking for is accomplished. On lingerie you would just trim the material next to your stitching and you have a fast cute hem. I use this technique on children's knits and then just cut off the folded hem and you have a fast shell delicate hem, almost like a shell rolled hem (easy and fast). You can also do this technique on a neck, sleeveless shirt, etc. by folding the edge in say 3/8 to 1/2 inch and scallop (shell stitch) the folded edge and then trim.
On the gecko t-shirt reviewed here on PR, I had my folded interlock strip and attached it as a ribbing to the inside of the neck, once attached I then use the shell stitch (after practice so it is correct) and stitched around the folded edge of the band. I then fold it down again over the outside of the shirt and stitched over previous shell straight stitches to secure the band down. The inside is also finished nice because you are capturing the raw edge in the fold. I take the steam iron and shape while pressing for the neck. I hope this makes since. You can message me anytime if you need more help. Just experiment!! Play with the material and stitch, you will develope your own technique, very cool and fun to do!
Here are two more pictures of the shell stitch used for a hem and neck finish on a little girls t-shirt. This is where I just folded under a little, shell stitched and cut! Cute and fast :) Sorry I didn't know how to hyper link additional pictures.