Flat-Fell Seam
The flat-fell seam is frequently used on sportswear, menswear and reversible garments.
With the wrong sides of the fabric together, stitch a plain seam. Open the fabric so that the wrong sides are no longer together. Press the seam allowances to one side, so that one seam allowance lays on top of the other.
a) Trim the underneath seam allowance to 1/8" (3mm).
b) Turn under 1/4"(6mm) of the top seam allowance and baste it in place over the trimmed edge. (For quick sewing, use pins or glue stick instead of hand basting.)
c) Edgestitch close to the fold.