What Is a French Seam?

What is a French seam used for?

A French seam joins two pieces of fabric and finishes their edges in the same construction, sealing the raw edges inside a narrow fold so they cannot fray or show. A plain seam in woven fabric usually needs a separate finish, such as overlocking, especially where the seam is exposed. A French seam needs none, because nothing is left exposed.

Sewing guides recommend it where seams are on view, as in sheer cloth or unlined garments. They describe it as strong and durable, able to take a lot of wear and laundering.

How is a French seam sewn?

A French seam is sewn in two passes: first with the wrong sides of the fabric together, then, after trimming, with the right sides together, so the second seam traps the first inside it. The method in the University of Kentucky’s extension guide, in four stages:

1. Place the two pieces wrong sides together, the reverse of a normal seam, and stitch about 1 cm (3/8 inch) from the cut edge. Press.
2. Trim the seam allowance to 3–6 mm (1/8–1/4 inch) and press it open.
3. Fold the fabric along the stitching so the right sides are together and the cut edges are covered. Press the fold sharp.
4. Stitch again about 6 mm (1/4 inch) from the fold, enclosing the raw edges. Press flat, then to one side.

With these numbers, the final line of stitching lands about 1.6 cm (5/8 inch) from the original cut edge. That matches the guide’s default seam allowance of 5/8 inch, so a French seam sits exactly where a plain seam would. Other guides differ: a Utah State University Extension sheet trims to 3 mm and sews the second pass at about 1 cm; an Ohio State University Extension sheet sews 6 mm first, then about 1 cm.

Whatever the figures, the trimmed edges must fit inside the second seam; the Utah State sheet stresses “precise stitching and careful pressing and trimming”. The finished ridge holds four layers of cloth: the two trimmed edges, wrapped in a fold of both garment pieces.

How do you spot a good French seam, and a poor one?

Turn the garment inside out: a good French seam is a narrow, even ridge of folded fabric, pressed flat to one side, with no raw edges, fuzz or thread loops anywhere along its length.

Checks to make with the garment in your hands

• Look at the inside. You should see a closed fold of the garment’s own fabric. Loops of thread wrapped over the edge mean an overlocked seam.
• Look at the outside. A French seam shows a single seam line, like a plain seam. Two parallel rows of topstitching on the right side point to a flat-felled seam, or a look-alike such as a welt seam, instead.
• Run a thumbnail along the free edge of the ridge. A true French seam has one clean fold there and no stitching. A row of stitching close to that edge, joining two folded edges, marks a mock French seam.
• Measure the ridge. Sewing guides place the second seam about 6–10 mm (1/4–3/8 inch) from the fold, so the finished ridge is roughly that wide. A ridge much wider than about 1 cm is outside the figures these guides give.
• Hold a sheer seam up to the light. It should read as one even band. The Utah State sheet’s list of “Industry Standards for Well-Constructed Seams” includes even width throughout, no puckers or twists, and a seam that is smooth and lies flat.

Red flags

• Fuzz or loose threads escaping along the fold: the raw edges were not fully enclosed.
• Puckers, twists or a ridge that wanders in width.
• A French seam on a knit or stretch fabric. The guides describe it for lightweight wovens and recommend serged or stretch-stitched seams for knits.

Do not mark a garment down because its armholes use a different finish. The Kentucky guide recommends a mock French seam for the sleeve and armhole area of a sheer, special-occasion garment.

What a French seam tells you

Every French seam was stitched twice, with trimming and pressing in between. An overlocked seam, by contrast, is stitched and finished all in one process, the Kentucky guide notes. A French seam shows care at the seams; it says nothing about the fabric, so judge that separately.

Where do you usually find a French seam?

French seams turn up mainly in sheer, lightweight and delicate woven fabrics, including blouses, dresses and lingerie, and in unlined garments whose seams are on show.

• Sheer and lightweight cloth. Fabrics such as chiffon, organza and georgette, where any seam shows through.
• Lingerie and sheer blouses. Ohio State University Extension recommends the French seam for lightweight or transparent fabrics, “as in sheer blouses, dresses, or lingerie”.
• Unlined garments. Where there is no lining to hide the inside, Utah State recommends French seams for visible seams, including those in unlined jackets.
• Historic dress. The V&A holds an evening dress of machine-made silk net, made in Great Britain in 1820–1825 and “hand stitched with turned French seams”. An English cotton day dress of 1805–1810 there has a “French seam at inside front”.

Guides point knits and heavier cloth towards other seams. The Kentucky guide recommends a 3-thread serged seam for knits where stretch matters, bound finishes for medium to heavy wovens, and the Hong Kong seam, also called a Hong Kong finish, for coats and jackets.

French seam vs Hong Kong seam, overlock and flat-felled seam

A French seam hides raw edges in a fold of the garment fabric; a Hong Kong seam binds each edge in a separate strip, an overlock wraps them in thread, and a flat-felled seam folds them under two rows of stitching that show outside.