Instructables
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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.
 
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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

Picture of Stitch seam as usual
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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.
debflowers2 years ago
This is called a french seam in tailoring. :)
creates a flat finished, and cleaner looking seam. Usually found on jeans and tailored clothing, and yes it is used to prevent frayed edges.
rachel (author)  debflowers2 years ago
A french seam is slightly different, it's like a flat-felled seam that isn't stitched down. It is made by first stitching the seam with the edges matched up, but the WRONG sides together, and a very small seam allowance (a quarter inch or less). Then the fabric is turned and folded the other way, with the RIGHT sides together, as with a normal seam. The seam is then sewn again with a half-inch or so seam allowance, with the first seam caught inside. It makes a beautiful inside finish!
Cross_4 years ago
It looks like Variants 1 and 2 don't enclose the seam, relying instead on overcasting to prevent fraying. This kind of defeats one of the purposes of the flat felled seam I think.
rachel (author)  Cross_4 years ago
That's true, but very often the seam is used for the look and strength, and less for the anti-fraying properties. These last two are not true flat-felled seams but they are easier and faster if you don't have a flat-felled machine, which is often a worthwhile tradeoff.
jauncourt5 years ago
This is such a perfect explanation of how to do this that I linked to this instructable from mine instead of trying to write out the instructions.
woot! for the fabric choice! looks great rachel. well done!
golden star5 years ago
Thanks for this great tutorial. I am thinking of using flat felled seams for making some bags without lining and this is a way to actually finish the seams.
rachel (author)  golden star5 years ago
That's an excellent use for the technique. Post an instructable about your bags, or at least pictures!
troyml5 years ago
You've made it very clear, thanks. What distinguishes topstitching thread from 'regular' thread, and why would I want to use that instead?
rachel (author)  troyml5 years ago
It is just thicker, so it is more visible. It's also stronger, but unless you also put it in the bobbin this isn't significant (and 3 rows of regular thread are plenty strong). Regular thread is smaller in diameter than the warp & weft of most denim so it tends to sink into the weaving and become hard to see. Topstitching thread sits on top producing a nice look. There are actually lots of different weights of thread but most are only available to manufacturers. In well-stocked fabric stores you can usually get up to 3 weights: regular, quilting (slightly thicker) and topstitching (thickest). Sometimes there is a kind of fluffy decorative serger thread as well which I have forgotten the name of. This is entirely aside from the content of the thread, which may be of several different fiber types, the most common being 100% polyester and cotton-wrapped polyester. Also usually available is 100% cotton (for sewing silks). Sometimes you can find silk thread, usually this is used for embroidery.
This is a really great explanation! :D