French seam vs flat-felled seam: which to use where

A French seam hides the raw edges inside a narrow second seam, so it is the choice for sheer, light and fine wovens where any finish would show through. A flat-felled seam wraps one allowance around the other and topstitches it flat, which makes it the strongest and flattest seam for denim, canvas, twill and shirting. Use French seams on blouses, slip dresses and lightweight linings, and flat-felled seams on jeans, work shirts, bags and anything that is washed hard.

What is a French seam?

A French seam is sewn twice. The first pass joins the pieces wrong sides together with a small allowance; the allowance is trimmed, pressed to one side, and the pieces are folded right sides together along the seam. The second pass encloses the trimmed edges inside a tube of fabric. From the outside the seam looks like any plain seam, and from the inside you see a neat fold about 6 mm (1/4 in) wide with no raw edge anywhere.

Because nothing is cut away from the seam after the second pass, a French seam is also the safest finish on fabrics that fray at a touch. Loosely woven linen, silk chiffon and viscose crepe all shed threads from a plain seam, and a French seam seals them in.

What is a flat-felled seam?

A flat-felled seam is the seam on the outside leg of a pair of jeans and down the side of a classic work shirt. The pieces are sewn, one allowance is trimmed to half, the wider allowance is folded over the narrow one and the fold is topstitched down. The raw edges end up wrapped inside the fold, held by two rows of stitching: the original seam and the topstitching. Some sewists add a third row close to the seam line for the full jeans look.

Sewn with the wrong sides together, the fold and its topstitching sit on the outside of the garment, which is the traditional look. Sewn right sides together, the fold is on the inside and only one row of topstitching shows outside, which is quieter on a shirt.

How do you sew a French seam?

Start from a standard 1.5 cm (5/8 in) seam allowance. Both passes together use it up.

1. Place the pieces wrong sides together and sew 6 mm (1/4 in) from the raw edge. This is the opposite of a normal seam, which is the step most people get wrong once.
2. Trim the allowance to 3 mm (1/8 in). Trim evenly: any stray fiber longer than the final seam will poke through the second pass.
3. Press the seam flat as sewn, then press it open and fold the pieces right sides together exactly along the stitching line. Press the fold sharp.
4. Sew 9 mm (3/8 in) from the folded edge. The first seam and its trimmed edges are now inside.
5. Press the finished seam to one side, usually toward the back of the garment.

On very fine fabric, reduce both passes to 5 mm and 7 mm (3/16 in and 1/4 in) for a narrower, lighter seam. On anything thicker than a quilting cotton, the enclosed layers start to build a ridge that shows after pressing.

How do you sew a flat-felled seam?

1. Sew the seam at 1.5 cm (5/8 in), wrong sides together for an outside fell or right sides together for an inside fell.
2. Press both allowances to one side. Trim the lower allowance to 6 mm (1/4 in).
3. Fold the upper allowance over the trimmed one, tucking its raw edge under so the fold lands on the stitching line of the first seam. Press, and pin or clip the fold every 5 cm (2 in).
4. Topstitch close to the folded edge, 2 mm (1/16 in) from the fold, through all layers. Use a longer stitch, 3 mm (about 8 stitches per inch), and a topstitching or jeans needle on denim.
5. Press again. For the jeans look, add a second row 6 mm (1/4 in) from the first.

A felling foot folds and feeds the allowance for you and is worth having if you sew several pairs of jeans. For one project, a good press and pins do the same job more slowly.

Which seam is stronger?

The flat-felled seam, clearly. The load on the seam is carried by two or three parallel rows of stitching, and every raw edge is locked inside a fold that cannot ravel. That is why workwear, jeans and sailcloth have used it for over a century, and why sewing with denim almost always means felling the main seams.

A French seam is strong enough for the garments it belongs on. The first pass carries little load, the second pass carries the seam, and the enclosed edges cannot fray. It is not the seam for the crotch of trousers or the side of a tote bag, both because of the strain and because thick fabric does not fold into it cleanly.

Which fabrics suit each seam?

French seams suit anything you can see light through and anything light enough to roll twice without bulk: chiffon, georgette, organza, voile, batiste, lawn, silk crepe de chine, lightweight linen and viscose. They are also the usual choice for unlined blouses and dresses in these fabrics, and for the side seams of a bias-cut slip dress, where a serged edge would print through on the outside.

Flat-felled seams suit stable, medium-to-heavy wovens with little stretch: denim, canvas, duck, twill, chambray, poplin, oxford and corduroy. They also work on quilting cotton for a sporty finish. They are a poor choice on knits, which stretch under three rows of straight stitching, and on fabrics that mark when pressed, because the fold is pressed hard several times.

Can you use French or flat-felled seams on curves?

Both seams prefer straight lines and shallow curves such as a side seam, a shoulder seam or a long skirt seam. On a tight curve the enclosed allowance has to be shorter or longer than the seam it is pressed against, so the fold puckers.

Armholes can be French-seamed on fine fabric if the first pass is trimmed to 3 mm and clipped lightly, and the second pass is sewn slowly. Many sewists prefer a narrow serged or bound finish on the armhole of a sheer blouse and French seams everywhere else. A flat-felled armhole is standard on work shirts, where the sleeve is set in flat before the side seam is sewn and the fell runs around the gentle curve of a dropped shoulder.

Tip: Sewly's patterns include a 1.5 cm (5/8 in) seam allowance on woven fabric by default. Both seams here are drafted around that figure, so you can sew them without changing anything; if you prefer to fell with a wider fold, set the allowance to 2 cm (3/4 in) before printing.

Which should you choose?

• Sheer or lightweight blouse, dress or slip: French seam. Nothing shows through, and the fabric is light enough to fold twice.
• Jeans, work trousers, chore jacket: flat-felled seam, outside fell with two rows of topstitching, on the outseam and the yoke at least. Fell the inseam too if your machine handles the bulk.
• Shirt in poplin, chambray or oxford: flat-felled seam, inside fell with one visible row, on the side seams and the sleeve seam.
• Unlined linen garment: French seam on lightweight linen; flat-felled on medium or heavy linen, which frays badly and benefits from the enclosed fold.
• Tote bag or apron in canvas: flat-felled, for strength and a clean inside with no lining.
• Anything in a knit: neither. A stretch seam with a serged or zigzag finish does the job, as in finishing seams without a serger.

When in doubt, sew a 15 cm (6 in) sample of each on scraps of the actual fabric, press them and hold them to the light. The one that lies flat and stays invisible from the outside is the one to use.