How do I sew a French seam?

A French seam is the neatest finish available on an ordinary machine, and it is the only one that leaves no raw edge anywhere — which is why it is the standard on sheer fabric, on unlined light garments and on anything where the inside is as visible as the outside. The technique confuses people once, because it begins with the pieces wrong sides together, which is the opposite of every other seam.

What you'll need

• Light or medium woven fabric — this is not for thick cloth or knits
• An iron and ironing board
• Sharp scissors for trimming
• A point turner or a blunt knitting needle

Step by step

1. Work out how to split the allowance.The two seams together must equal the allowance the pattern expects. With a 15mm allowance, sew the first seam at 5mm and the second at 10mm. With a 10mm allowance, 4mm and 6mm. The second seam must be comfortably wider than the trimmed edges it has to enclose.
2. Place the pieces WRONG sides together.This is the step people get wrong, and it is worth saying aloud as you pin. The right sides face outwards. Pin and sew the first seam at the narrow allowance.
3. Trim the first allowance hard.Down to about 3mm, evenly, with sharp scissors. Any stray whisker left long will poke through the second seam and show on the outside, so trim more carefully than feels necessary.
4. Press the seam open, then to one side.Pressing it open first sets the stitches and makes the next fold accurate. Then press both allowances to one side ready to turn.
5. Fold the pieces right sides together on the stitch line.Turn the work so the pieces are now right sides together, and roll the fold between your fingers until the first stitch line sits exactly on the crease — not a millimetre inside it. Press hard. This roll is what decides whether the seam is invisible from the outside.
6. Sew the second seam at the remaining allowance.Measured from the new folded edge. The trimmed raw edges are now inside a channel and must be completely enclosed by this row of stitching. If any whiskers appear beyond the stitch line, the second seam was too narrow.
7. Press the finished seam to one side.Usually towards the back on a garment. The seam has a definite thickness and will not press flat like an open seam, which is normal.
8. Check from the right side.You should see a single plain seam line with nothing showing. A ridge or a shadow of the first stitching means the fold was not rolled far enough — unpick the second seam, re-roll and re-press rather than living with it.

Tips

• On a very sheer fabric make both seams narrower still — 3mm and 5mm — so the finished seam is a fine line rather than a visible band.
• Use a shorter stitch than usual, about 2mm, for both rows. It gives a cleaner fold and a firmer enclosure.
• If the fabric frays as you trim, sew the first seam, trim, and press immediately rather than leaving the trimmed edge to be handled.
• A rolled-hem foot is not a substitute. It makes a narrow hem, not a seam.

Common mistakes

• Starting with right sides together — You then end up with the raw edges on the outside of the garment after the second seam. There is no recovery except unpicking both rows.
• Not trimming the first allowance enough — The untrimmed whiskers escape past the second stitch line and show on the right side as fuzz along the seam.
• Using it on a curved or thick seam — The enclosed allowance will not turn round a tight curve without pleating, and in thick fabric the finished seam is a hard rope. Use a bound or flat-felled finish instead.

If it doesn't work

Questions people ask