Two curved pieces sit on the table, one edge bending outwards and the other bending in, and the instinct is always the same. Make them agree first. Ease one round the other, pin along the whole length, check the ends, add more pins, and only then take the thing to the machine. That instinct is the source of most of the trouble curves cause, because it asks two edges that curve in opposite directions to lie together along their entire length, which they will not do while they are flat.
The alternative is to stop asking. Lower the needle, sew a short run, stop with the needle still down in the fabric, and use your hands to bring the next stretch of the upper edge into line with the piece underneath. Sew that stretch, stop again, adjust the next one. At no point in the whole seam is the curve aligned as a whole. It is aligned only at the place the needle is about to pass through, which is the only place alignment ever mattered, and that is precisely why the pins turn out to be unnecessary.
Here is what each part of it is doing:
- Aligning the whole curve before you sew is what creates the pleats. When two opposite curves are forced to match along their full length while lying flat, the difference between them has to go somewhere, and it goes into small bubbles of excess between one pin and the next. Those bubbles are not a fault in your pinning. They are the geometry of the two shapes objecting to being made congruent on a flat table, and each one is a pleat waiting for the needle to sew it in permanently. Aligning in short runs instead keeps that excess ahead of the needle, where it is still loose and can still be moved. Behind the needle the question has already been settled, ahead of it nothing has been decided yet, and the work is only ever happening at the boundary between the two.
- The needle down is the thing that makes any of this possible. Stopping with the needle buried in the fabric holds the work at exactly the point that matters and nowhere else. Your hands come off, the next stretch of edge gets lifted and turned and brought into line, and the seam you have already sewn does not move a thread while you do it. Take the needle out before you let go and you lose the registration between the layers, which is how the piece ends up drifting further with every adjustment. If your machine can be set to finish each stop with the needle down, set it. If not, turn the handwheel until the needle is through the fabric before you lift the presser foot, every single time, until it stops being something you have to remember.
- The upper piece goes in set back from the raw edge of the lower one. The first stitches look wrong, and that is expected. Because the two edges curve against each other, laying them flush at the start puts the seam allowance out of register as soon as the curve begins to take effect, and you spend the rest of the seam chasing an edge that has run away from you. Feeding the top piece slightly inside the edge of the bottom one gives it the room it needs to swing round as the machine pulls the layers through, so the two raw edges arrive level at the needle itself. Judge the offset by where the edges meet under the foot, not by how tidy the two pieces look together before they get there.
- The ripples and creases that appear as you sew will press out. A curve is being persuaded into a straight line of stitching, so the fabric gathers itself behind the foot and the seam looks puckered and unpromising while it is still on the machine. Nothing has gone wrong. That gathering is the fullness of the curve arranging itself, and the iron takes it out. Resist the urge to stop and unpick on the evidence of what you can see under the needle, because that evidence is misleading at that moment. Sew to the end, press the seam properly, and then look. The seam you judge should be the pressed one.
- Make sure the very end of the curve is fully caught in the seam. The last stretch is where a curved seam most often fails, because the piece has the least fabric holding it there and both edges are trying to slip away from each other at once. As you come up to the end, check that both raw edges are genuinely going under the needle, and if catching them both means taking a fraction more than the allowance you planned, take it. A little extra depth at the end of a curve will never be noticed. An edge that was not caught turns into a loose flap or a gap right at the join, and it will only announce itself once the block is together and considerably harder to put right.
Watching the stop and adjust rhythm run through a whole curve makes the pace of it clear, and it is slower and calmer than most of us expect:
What changes here is not your control over the fabric but where you decide to apply it. Trying to hold an entire curve in agreement is a job that cannot be done, and the pins were only ever an attempt to do it anyway. Holding a short stretch in agreement is a job your hands are extremely good at, and it happens to be the only stretch that is about to be sewn. Curves stop being the seam you put off and become the seam you sit down to on purpose, which is a quiet sort of freedom to be given by a needle staying where it was.
