Remember the plain seam which is sewn right side to right side. This means that the wrong sides are on the outside and the seam is thus sewn on the wrong side; So when you are finished sewing, the seam with its seam lips on both sides is on the inside or wrong side and this is what you want. You want the seam lips on the inside so that all that is seen on the outside is a line of stitching.

For the flat fell or fell seam, you begin by sewing your first seam on the inside or wrong side of the fabric. Thus fabrics are right side to right side. This seam should be approximately 3/8 inch and then you snip off one of the seam lips to within 1.8 inch of the stitching line. Then you turn the larger lip in under the smaller one. You aim to enclose the smaller lip, Note that there are three layers. You can ask if you just want to sew the two upper layers together i.e. the two seam lips or do you want to sew the seam lips down to the actual fabric.

This is an interesting move i.e. just to sew the two lips together through hand stitching and this is an elegant finish. The lips will hold together; and the seam will have a turned in look. This is the fell or flat fell seam and this seam is now usually done by stitching the three layers together through a top-stitch. In this way the flat fell is done by holding down the seam lips to the fabric itself; the seam lips having been enclosed in each other.

Then for the French seam, the difference is that you start off at the right side of the garment. Thus the sides are placed wrong side to wrong side and the seam is sewn. You can then see the stitching line on the wrong side. The seam lips are then shortened to approximately one eighth of an inch. The garment is then turned around so that you are working on the right side. The aim is now to enclose the inside seam lips. Sew then at least inch from the fold to enclose the inside lips. This stitching line should not be visible on the outside. What is done here is not a seam at all. It is a stitching line done to enclose the inside lips and to neaten the seam. French seams are an elegant finish for fine and lightweight material such as chiffons and light fine silks.