The material you are refering to that is cut on the bias is called "listing" avaiable in 1 1/2" or 2" material. Listing is used to run headliner rods, seat wires, and other types of fasteners through. You hit the nail on the head that most trimmers do not use a backing. My point and case. When done correctly as I stated, all "professional" upholsterers will install a backing material on all french seams, that is what makes a true french seam. This makes all three stitches load bearing. If you do not have a backing material for a french seam you are only relying on the center stitching to do all of the load bearing. This is called a standard single blind stitch that is as basic as you can get, and in no way compares to the french stitch or the top stitch. The french seam is sewn through 3 layers of material ( 2 covering / 1 backing ), your actual seat cover will stretch and give out before the seam will. I hate to be harsh, but if most trimmers / upholsterers in your area do not use the backing on a frence seam, time to look elsewhere for a qualified person that will make sure the customer gets what they paid for.
