This is the ultra-thin version of the French Seam. It’s very useful if you’re making fine linen pieces (like coifs), or if you’re working with dolls and cannot divorce yourself from the idea of fully finished seams….
The size of a Hairline Seam (right) compared to a normal French Seam (left).
Seam Allowance Required: 3/8-1/2″
With WRONG sides together, sew at 1/4" from the pattern's seam allowance.
Sew a second line of stitch right outside the first. (Literally 2 threads away.)
Completed double stitching.
Trim the seam allowance very close to the stitching.
Fold the fabric back around the seam allowance. Press.
Sew, encasing the seam allowance as closely as possible without sewing into it.
The finished Hairline Seam. Like the French Seam, it's basically invisible from the front.