When sewing with woven fabrics there are times when a seam just wants to stretch out and won't stay put....now keep that word "stay" in mind while I explain...
Today I am making the lovely and fashionable Fifth Avenue dress pattern
and because the fabric is a bit thick, to keep down the bulk I decided to use organza in place of the traditional interfacing the pattern calls for.

With Organza being an
actual woven fabric, that little dip in the sweetheart neck was going to be prone to stretching, so I used a "stay" to stabilize that portion of the neck. This method can be used any place you want to prevent stretching on any seam, such as shoulders, waist line, necklines, pockets, and can even ease in a seam that has already stretched.
Pin your hem tape to match your seam line, yup, right down the middle, and cut to match.
At both ends of the tape I mark and X with pencil where the seam lines will cross,
Sewing with the tape toward you, and the garment to the feed dogs, stitch the tape down the middle over your previously sewn seam, making sure the cross points match at the connecting edges. This is the only way to be sure that the garment matches the pattern sizing exactly. Clip, trim, and understitch as usual. Now your seam will "stay" where you want it!
"Stay" is a good name for it, don't you think?
Have a wonderful sewing day!
Laura