Flat-felled seams are used for strength (and decoration) on nearly all jeans and denim clothing. They're also often used on tents, outerwear, and anything where you really don't want a torn seam, or frayed edges. A true flat-felled seam can be made on a home sewing machine, although in manufacturing a special machine is used that only does this kind of seam.
It's slightly arduous to do the true kind without the specialized machine, though. I usually use a similar seam that's nearly as strong and a lot easier, which looks the same on the outside of the garment.
You'll need regular thread in the bobbin, and two spools of thread in the top of the machine: one regular for the construction stitch, and one of heavier topstitching thread for the decorative topstitching. Note that there's no functional reason not to use the same thread for the topstitching, but the heavier thread looks nicer. In this instructable I've used orange thread in the bobbin, white for the regular thread, and light green for the topstitching, so they are easily distinguished in the pictures.
Step 1: Cross Sections
A flat-felled seam is one where the two fabric edges are wrapped around each other such that each raw edge is encased in a fold of the other. This is easier to look at than to describe!
The first pic is a cross section of a true flat felled seam. The second is the cross section of the alternate version (both alternates have identical cross sections). If your sewing machine doesn't like heavy weight fabrics, I recommend the alternates not only because they're easier, but because they produce fewer thicknesses of fabric to sew through.
Step 2: Stitch Seam As Usual
The first step in any of the versions is to stitch the seam to be felled, right sides together, as usual. Use a 5/8th inch seam allowance (this is standard on home sewing patterns) and regular thread.
Stitch as precisely as you can, for best results. This is pretty easy on denim which is stiff enough that it does not shift as you sew.
Step 3: True Flat-felled: Press
Trim one of the seam allowances to 1/4 inch, being careful not to cut into the other. Press the larger seam allowance over the trimmed one. Then open the seam on the garment side and press both seam allowances to the trimmed side. Be as accurate as you can, since you'll be sewing on the other side. You won't be able to see this side and you'll have to rely on the distance from the seamline being consistent.
Step 4: True Flat-felled: Topstitch
Run a second line of stitching a quarter inch over from the first line, or as far over as you like, keeping in mind that you don't want to either:
- stitch off the edge of the folded-under seam allowance
- stitch so far away from the underneath fold that it comes unfolded
Step 5: Alternate 1: Serge and Topstitch
If you have a serger, instead of the trimming and pressing, you can simply serge off a quarter inch of your seam allowance, leaving 5-8th inch of serged edge. Press to one side and stitch in the same manner as the true flat-felled seam.
Step 6: Alternate 2: Zigzag and Topstitch
If you don't have a serger, you can use a zigzag stitch instead. Trim both seam allowances to 3/8 inch and zigzag stitch along the edge using a short, wide stitch. Again the topstitching is the same as for a true flat-felled seam.
Note that if you think far enough ahead, you can adjust your pattern to reflect the 3/8 inch seam allowance instead of using 5/8 and trimming it down after stitching. Assuming you know you won't have to alter the finished garment outwards, of course!