What Is a French Seam?
It is used most commonly on very sheer fabric so the seam blends with the fabric. It is well suited for sheer fabric in place of a zigzag or serge stitch that would leave a lot of thread visible through the fabric.
A french seam is most common on a straight seam. It is difficult to use on a curved seam but can be done if you use very small seam allowances. Use scraps of the fabric to practice a curved seam before sewing on your actual item.
Although a French seam is most commonly used on sheer fabric, it can be used on any fabric to enclose a seam allowance and keep a soft edge to the seam. This works especially well for a person with very sensitive skin or children who are bothered by rough seams.
A French seam also is commonly used to join sections of fabric when making curtains or window treatments for a seam that blends into the fabric and is visually appealing from the wrong side of the curtain which is visible through the glass window.
The first step is sewing so the seam allowance is on the right side of the item. This is the opposite of how you normally sew a seam.
If your pattern directions call for you to use a standard 5/8-inch seam allowance, create a 1/4-inch seam with the wrong sides of the fabric together. Accuracy is important. If you change the seam allowance and you are sewing a garment, you will be changing the way the garment fits when you are finished. A change in the seam allowance by 1/4 inch in four seams will add up to a 1-inch difference in the finished item.
If you are joining sections of fabric for something, such as curtain panels, always make the first seam allowance smaller than the next seam allowance so the edge of the seam allowance won't protrude from the finished French seam.
Press Every Step
Press the seam as it was sewn.
Fold the fabric along the seam line, so that the right sides of the fabric are together.
Press well, so that you are working with a sharp crease at the fold on the first seam. If necessary, use a pressing tool such as spray sizing or spray starch to give the fabric a sharp or firm edge, so the fabric will stay folded as you sew the next step. This will come out in the laundry if you are creating a soft seam for a person with sensitive skin.
If the fabric is slippery and wants to slide around, pin the seam so that the sewn seam stays at the edge and the fabric doesn't distort as you sew the seam.
Enclose the Seam and Sew Down
For a 5/8-inch seam allowance, sew a new seam 3/8-inch from the folded seam edge.
For other seams, be sure this seam is enclosing the first seam so that none of the seam allowances from the first seam sticks out through the seam when you are finished. This is very important because those threads sticking out would end up showing on the right side of the item.
To finish, press the seam as it was sewn and from the right side of the item.
If the fabric you are sewing is fraying as you sew, take your time and tuck those wandering threads inside the seam as you sew or trim them as you sew. Do not pull the fraying threads because you will probably make the fraying worse.