I sewed my best French seam yet today, in a crinkled silk chiffon. Rather than show it off, I figured I'd take pictures along the way to show how one is done.
A French seam is a method of seam finishing used especially for sheer or fine fabrics that may fray easily, or for garments that want a couture inside finish. For more commercial garments, serging is used (look inside your t-shirt or jeans). But inside of an evening gown, you will see seams like the French or Hong Kong finish, for a more elegant treatment.
French seams are easy in theory, but difficult in practice. The basic idea is a set of two seams: one to create the seam, and a second to capture the raw edge of the original seam allowance. Here is how to sew one!
Your first step is to sew your fabric wrong sides together. Later, you will be flipping it so it will be right sides together. When I first learned French seaming I did this backwards all the time. Calculate your seam allowance carefully - if your pattern allows for a half inch, realize that your final seam will probably be about 3/16" so sew your initial seam minus that final seam measurement.
After you sew the initial seam, press the seam flat (this is a wonderful trick - before opening up the seam to press open the seam allowance as usual, run the iron straight up the stitch line. This helps balance the stitches, so you will never end up with a rippled seam.) Then, while the seam is still closed, trim away the seam allowance as close as possible - usually 1/8" will do. Then open up the seam and press.
Now you will sew your fabric right sides together. Fold the fabric with your fingers (first picture) so that the seam line is directly on the fold line, and pin. Rather than trying to iron it this way first, I find it is easier to iron it completely flat and then bend at the stitch line - it will bend quite easily and naturally here. Stitch 3/16" away from the fold. Your trimmed seam allowance is 1/8", so if you stitch 1/16" away, you trap it inside the stitched channel so fraying is prevented, and a beautiful clean finish is created.
I find this works easiest in sheer fabrics - not because they are easier to sew, but because you can see your trimmed seam allowance and know exactly where to guide the foot. In the second picture, you can see how my needle mark is lined up just outside the trimmed allowance.
The other benefit to sheer fabrics is you can check yourself as you go - hold the fabric up to the light, and make sure your seam allowance doesn't travel beyond your stitch line. If it does, when you press the final seam you will have little "fuzzies" escaping the seam. These look very awful! If working with an opaque fabric, do not fret - simply make sure your measurements are perfect, and you should be fine.
When you are finished stitching, balance and press your stitch line once again. You should have a seam with what appears to be a very thin pin-tuck running the length of your fabric, on the wrong side.
Turn your fabric to the right side and press your seam open like any normal seam. The result (on the outside) should be a perfectly regular seam (unless your fabric is sheer, in which case you will see a very fine and delicate line of allowance). On the wrong side, you have the enclosed seam that looks like a pin-tuck. Check for fuzzies and trim if you can. If not, you may need to go back and adjust your stitch line.
And you are done! French seams are truly beautiful and take a good amount of concentration and patience - but they are worth it. I personally love being able to see them through sheer fabrics, because to me they are like a thin skeletal outline of the construction of the garment (like pencil lines on a sketch). They also give the inside of a garment the effect of being as nice as the outside. After all, it's the inside that counts.