Why does my machine stitching come undone at the ends of a seam?

A machine seam is two threads knotted together once per stitch, so a break in the middle does not run. The ends are different: the last knot has nothing beyond it, so a pull on the thread tail can walk the stitches back one at a time. Every seam therefore needs its two ends locked, and the right way to do it depends on what the seam is for.

Step by step

1. Start with the needle down in the fabric.Lower the needle into the exact start point by handwheel before lowering the foot. Starting with the needle in the air lets the first stitch pull the fabric down into the needle plate and makes a tangle.
2. Hold both thread tails back behind the foot.For the first three or four stitches. Loose tails in front get dragged down into the bobbin race, which is the real cause of most start-of-seam nests.
3. Backstitch to lock a structural seam.Sew forward three or four stitches, press and hold the reverse lever, sew back along the same line, release and carry on. Three or four stitches is enough — a long backstitched section adds bulk and is harder to unpick if you need to.
4. Backstitch at the end the same way.Stop with the needle down at the seam end, reverse three or four stitches, then forward again to the end. Ending with a forward run leaves the threads at the edge where you want them.
5. Use a shortened stitch instead where a backstitch would show.On fine or sheer fabric, or a visible topstitched line, drop the stitch length to about 1mm for the first and last centimetre. The stitches lock each other by sheer density without a visible double line.
6. Use knotted tails on the finest work.Leave 10cm tails, pull the top thread through to the wrong side by tugging the bobbin thread until the loop appears, then tie the two together and thread the tails back into the seam with a needle. Slow, invisible, and right for silk and for hand-finished garments.
7. Do not backstitch where the seam will be crossed.If another seam will cross this one within the allowance, the crossing locks it. Backstitching as well just makes a lump under the presser foot later.
8. Trim tails close, but not flush at the very end.Leave two or three millimetres. Cut hard against the last stitch and the tail can be pulled back through the fabric, which unlocks the end you have just secured.

Tips

• Many machines have a lock-stitch or tie-off button that sews several stitches in place. It is neater than a backstitch on visible work and is worth finding on the panel.
• On a seam that will be under real strain — a crotch seam, an armhole, a bag strap — sew the whole seam twice rather than relying on longer backstitching.
• If the reverse lever on an older machine gives a stitch that does not land on the original line, sew the backstitch slightly slower and guide it; it does not have to be perfect, only overlapping.

Common mistakes

• Backstitching for two centimetres — It creates a hard, bulky, thread-dense section that shows through on the right side, is difficult to unpick, and is no stronger than three or four stitches.
• Not securing the ends at all because the seam is enclosed — It may be enclosed now, but hems get taken up, garments get altered, and an unsecured end that gets exposed will run.
• Snipping the tails flush against the fabric — The end knot needs a small tail to hold it. Cut absolutely flush and the last stitch can pull back through.

If it doesn't work

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