What is a mock French seam?
A mock French seam is a seam finishing technique entailing folding and topstitching the seam allowance layers inward to encase all raw edges. It is called a mock French seam because it resembles the structure and look of a traditional French seam. As opposed to a French seam application, a mock French seam is aligned and stitched in a conventional manner with fabric right sides touching- a French seam encloses the seam allowance edges by aligning and stitching the seam edges with fabric wrong sides touching then turning the layers back with fabric right sides touching (conventional seam alignment) to encase the fabric edges. An additional topstitch is then used to seal all fabric edges closed.
When sewing a mock French seam, prior to topstitching, the seam allowance layers should be folded at the same distance such that they align perfectly when touching. The easiest way to achieve this perfect alignment is to fold and iron each seam allowance layer individually at the same distance throughout- normally, with the fabric raw edges aligning to the seamline.