Wrap-and-tie styles often have a tie end passing through one side seam. It's not hard to handle this situation if the seam is pressed open flat—you just don't sew the part the tie end needs to go through. In principle, it seems it should be possible on a French seam too—just leave that gap unsewn on both passes—though there are some additional considerations. Here's one way I've done it.
1. The first French seam pass will be done wrong sides together, as usual, except for leaving the gap unstitched between the markings.
2. Press these first seam allowances open flat, pressing under the edges of the unsewn part the same amount. (You may want to trim off any fraying threads first, so those don't stick out and make hairy seams on the next pass. But that's no different from an ordinary French seam.)
3. To stop the edges of the unsewn area from fraying or getting pushed out of the seam later on, zigzag them down. You could also include bits of single- or double-sided fusible here.
4. Now make your second French seam pass, again leaving the marked gap unstitched. You can pivot and stitch perpendicularly across to the edge of the seam at each marking, if you like.
5. It's a pretty normal-looking French seam at this point, aside from that gap and zigzagged area.
6. Press the seam to one side, the side the tie end comes from. If the tie end is to come from inside the front of the garment and 'escape' to go across the outside of the back, then press the seam to the front. Or if it's coming from the back and emerges to go across the front, press the seam to the back. This will let the tie end follow a straight path through the seam, rather than having to zig and zag on its way out.
7. Now that the seam is pressed in the correct direction, push the top layer of the slit to one side, so you can edgestitch the bottom layer down to the outer fabric. Make this stitching a neat, even distance from the edge of the slit, as it will show on the outside. Or you can do this invisibly by hand.
8. Here's that stitching as it shows on the outside.
9. You can now stitch perpendicularly across the top and bottom of the slit for extra security, if you like, or make bartacks. Strictly speaking, this is less necessary if you pivoted across the seam in step 4.
10. Here's how it looks on the inside now that it's done. Not much different from step 5.
That's what I did with my Hollywood 1512 housecoat. Here are some things to consider if you're thinking about doing this:
- This approach is best used on fabrics that don't fray too much, because unlike the rest of the seam, the zigzagged edge will be subject to friction from the tie ends coming through.
- In step 3, if you have a wide enough first-pass seam allowance width to work with, in the opening area you could turn under the raw edge on each side and stitch it down, instead of zigzagging—you'd have to either make a tiny clip on each end of the opening (or just outside it) to allow the seam allowance to return to normal, or taper out the turned-under width to nothing. I've done this on one other project and it's worked well.
- Another (somewhat more involved) alternative to step 3 would be to apply a tiny facing on each side of the slit, just wide enough to cover each cut edge.
- Finally, it's slightly more awkward to pass the tie end through than it would be with a seam pressed open flat. You do get used to it, though.
So there you have it. Similar things should be possible on (true) flat-felled seams too (assuming, of course, you don't put topstitching through all layers of the opening); the main issue is controlling, stabilizing, and/or covering up the cut edges that would otherwise be completely enclosed by the seam finish, but for that opening.
