A type of seam commonly used for the inseams of jeans, and in the construction of tarps and tents and shade pavillions. It is much stronger than your average lay-two-pieces-of-fabric-together-and-stitch sort of seam.

Here's how to make a flat fell seam:

  1. Lay your two pieces of fabric together and stitch, but instead of putting the edges exactly together, have the seam allowance be a little more than twice as wide for one of the pieces. Also, unlike a normal seam, the front sides of the fabric should be facing out, and the back sides are facing each other.
    piece 1   ___________________________ ______
    piece 2   ___________________________|_______________
                                         ^stitch there
  2. Now fold the wider seam allowance over the shorter one. It might help to iron it in place.
    piece 1   ___________________________ ______\
    piece 2   ___________________________|______/
  3. Fold the whole top layer over the seam, and iron again.
                                ____________________ piece 1
                               / ______
                               \ ______\
    piece 2   __________________|______/
  4. Now position the fabric so what's shown above as the underside is on top as you sew it, and stitch through all the layers next to the fold:
                                ____________________ piece 1
                               / ____|_
                               \ ____|_\
    piece 2   __________________|____|_/
                                     ^stitch here
This will give you a nice flat seam with two rows of topstitching. If you only want one row of topstitching, you can start with the top sides of the fabric together, and the back sides facing out.

You can probably see why this type of seam is so sturdy.

Here's what the finished product will look like -- of course, the pieces of fabric will probably be much bigger:

|                 |
|     piece 1     |
|_________________| edge of the fold
|-----------------| second line of stitching
|-----------------| first line of stitching
|     piece 2     |

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