Let’s take a walk on the mathematical side of sewing and chat about circles!
“Wait, math and sewing?” you might ask.
Yes! A little bit of geometry can help unravel some of the trickiest parts of sewing, making those curved seams and hems so much easier to handle. Let’s dive in!
Have you ever sewn two curved pieces together and found the edges just wouldn’t line up, no matter what you did? Or struggled with puckering while hemming a rounded edge? Maybe you've turned out a curved seam and wondered why it won’t press flat? The answers all lie in the geometry of circles!
When you break it down, most curves in sewing are sections of circles. While they may vary in shape, they all follow the same principles. By understanding a bit about how circles behave, we can tackle curves with confidence.
Understanding Curves in Sewing
Before we begin, let’s define a couple of key terms—after all, if you're like me, it’s been a while since those school geometry lessons!
There are two main types of curves in sewing:
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Convex curves curve outward, like a hill.
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Concave curves curve inward, like a valley (or a cave—easy to remember!).
We also need to think about concentric curves and circles. This just means that as circles expand, their circumference (the measurement around) gets longer, and as they shrink, it gets shorter. Think of ripples on a pond: each expanding ripple is a larger concentric circle.
But, what does this have to do with sewing? Come along and see!
Seam Lines vs. Cut Lines
When a sewing pattern is created, pieces are drafted so that their seams match at the seam line, ensuring a perfect fit. But when seam allowances are added, the cut edges of curved pieces may not be exactly the same length due to the concentric nature of curves.
For example, when sewing two curved pieces together, like pocket pieces in the Meadow Skirt, everything matches up nicely because both curves are the same. But when sewing opposing curves together, such as setting a sleeve into an armscye, or attaching the lining and facing of the Grove Coat, things get trickier!
It's natural to want to match up the cut edges, but because one curve is slightly longer and the other is slightly shorter, the pieces won’t align properly that way. Instead, focus on matching at the seam line. To help with this, you might need to:
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Let the convex seam allowance ripple up slightly.
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Clip into the seam allowance of a concave curve to allow it to spread.
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Ease the fabric together gently, either as you work or with gathering stitches.
The key takeaway is to ensure that the fabric lays smoothly at the seam. You don’t have to worry too much about what the seam allowance is doing —as long as there are no tucks or folds at the seam itself, your garment will look beautiful!
Hemming Curves
Curved hems can be another challenge. Unlike a straight hem, where the fabric folds evenly, a curved hem behaves differently because of its changing circumference.
For example, on a convex hem (like you will find in the Flourish Dress Coat, Bloom Jacket, or Cascades Top), the cut edge is slightly longer than where you need to stitch the hem, making it tricky to fold evenly. A simple trick is to sew a guideline first. This line not only acts as a visual guide but also subtly gathers the fabric, making it easier to turn up and stitch smoothly.
For more extreme curved hems, you might need to make small snips into the hem allowance so it can spread and fold more easily. Just be sure to keep your cuts within the seam allowance so they remain hidden inside the finished hem.
Turning Out Curved Seams
Another common challenge is turning out curved seams smoothly. You’ll see this in many Twig + Tale patterns, but the hood base of the Nature Basket illustrates the concepts really well.
When turning a convex curve right-side out, the extra seam allowance inside creates bulk, leading to unsightly bumps. On a concave curve, the seam allowance is smaller than the outer edge, which can cause twisting and warping.
The solution? Clip those curves!
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On convex curves, trim little triangles out of the seam allowance to remove excess bulk.
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On concave curves, make small snips so the seam allowance can spread and lay flat.
With just a few simple clips, your curves will turn beautifully, giving your project a polished, professional look!
You can see this in action in this video - a part of our Nature Basket Sew-Along on Youtube:
Wrapping It Up
I hope you've found this little sewing-math lesson helpful - understanding how curves behave will help you sew with confidence, creating beautifully finished pieces with ease.
If you have any questions or want to chat more about sewing techniques, join us in the all-new T+T Treehouse community, or in the Twig + Tale Chat group on Facebook, Instagram, YouTube, and Bluesky as @twigandtale. We love seeing your creations and are always happy to help!
Happy sewing!
