A slot seam is considered a decorative seam. It is formed by two tucks which open into a slot backed with either matching or contrast fabric.
This seam requires some topstitching as well as the addition of an extra strip of fabric of your choice. The fabric strip adds a decorative element to the seam application.
To sew a slot seam:
1. Switch your machine stitch setting to a basting stitch or the longest stitch available on your machine.
2. Align the seam edges with face sides touching as you would with any conventional seam. Pin and machine baste the seam but do not backstitch neither at the beginning nor at the end of the stitch. This basting stitch will only be used for the construction process to keep the seamlines aligned while the slot underlayer is topstitched.
3. Clip the stitching at 1-inch increments using a seam ripper as shown.
4. Gently press the seam allowance open being careful not to pull the seam apart.
5. Cut a strip of fabric that is as wide as twice the seam allowance width. If the seam allowance is ½” like it is in this case, the fabric strip should be 1” wide (or more, if needed). It is up to you whether you choose matching or contrast fabric.
For a professional look, clean finish the fabric strip's raw edges.
6. Align the fabric strip underneath the seam such that it is centered in relation to the seamline, and each seam allowance edge matches each strip edge. Insert a few pins to keep the layers together.
Instead of using pins alone, it may be a good idea to hand baste all layers on both sides of the seamline. Doing so will keep the fabric strip perfectly aligned underneath during machine stitching.
7. With right side facing up, topstitch on each side of the basted seamline following an equal distance on each side.
Once topstitched, remove the hand basting stitches using a seam ripper.
8. Remove the machine basting threads that are keeping the seamline shut. The seam will now form a slot-like opening with the fabric strip peeking from underneath the two fold edges.
9. For a clean, professional finish, iron on top of the slot seam in the direction the seam was topstitched.
The end result should be a flat seam with evenly positioned topstitching and perfectly abutted folded edges.