Horsehair braid is not just for hems! You can also use it in a neckline to add shape and structure. This works especially well in strapless or strappy dresses, where the bodice is supported by boning. Sometimes interfacing the neckline isn't quite stiff enough to stand up to the structure of the bodice, especially when you want a lot of shape and stiffness around the neckline.
This red halter dress is a great example. As you can see above, it's boned around the bodice, but not over the bust. The horsehair braid keeps the neckline from collapsing above the bustline. It also keeps the neckline nice and crisp. I've seen it used in dresses like this pink chiffon one, where you need extra structure to support the ruching of the lightweight fabric.
To apply the horsehair braid, you'll want to stitch it to the lining neckline so that it's positioned just below the seamline. (Do this step after all lining seams are constructed and boning is sewn in.) Because my seam allowance is 5/8", I stitched my horsehair just a hair below that.
The braid can go all around the upper edge of the bodice, or stop at the side seams, which ever you prefer. If you have it go around to the zipper opening, make sure that you cut it just short of the opening so it doesn't get caught in any seam allowances.
Pull up the string in the horsehair braid so that it shapes to the curve of the bodice. It's a nice touch to stitch some scrap fabric around the short ends of the braid so the cut edges don't poke through the dress.
P.S. Happy Halloween!