The major difference in my opinion, is how the seam looks on the right side of the garment. A flat felled seam shows stitching on the right side (think of the seams on the side of a pair of jeans), while a french seam does not (it is found in many high end sheer garments). Thus, a flat felled seam tends to be found in sportier, more casual garments and a french seam is usually found in expensive evening clothes. Both types of seams are extremely sturdy and stand up to wear well. However, it can be difficult to get either seam to look great if the seam is highly curved - so I tend to use them for straight or slightly curved seams. I love both types of seams as I don't have a serger (yet) and it finishes off the inside of my clothing so that no raw fabric edges are visible.
Below, I have created a tutorial. I sure hope it makes sense - let me know if I have totally confounded y'all.
Flat Felled Seam
- Sew the two pieces of fabric, right sides together, with a 5/8" seam.
- Trim one side of the seam allowance down to 1/4".
- Fold the 5/8" seam allowance over and around the 1/4" seam allowance, completely enclosing the 1/4" seam allowance with the 5/8" seam allowance.
- Stitch down as close to edge as possible.
- This encases all raw edges inside the seam.
- Sew the two pieces of fabric, wrong sides together, with a 5/8" seam.
- Trim both sides of the seam allowance to 1/4".
- Press open the seam.
- Fold the fabric, right sides together, along the seam and press.
- Stitch 3/8" from the fold.
- This encases all the raw edges inside the seam.