Theory & Practice | Concave and Convex Curves

Grainline Studio | Theory & Practice | Concave and Convex Curves

So, I don’t know if this Theory & Practice thing will be a new feature but I thought it described perfectly what I’m about to explain in this post. I get a lot of questions about sewing the pockets on the Maritime Shorts. Specifically people want to know why the edges of the two pattern pieces aren’t the same length and how they are supposed to sew them together since they aren’t the same length. Basically this is a misunderstanding about sewing convex and concave seams together in regards to the outside edge of the pattern piece vs. the seamline. I apologize if this is super basic to you, but I get enough questions that I figured it wouldn’t hurt to address.

Grainline Studio | Theory & Practice | Concave and Convex Curves

Lets start with Convex vs. Concave shall we?

Convex : curving outwards (as illustrated in the small pattern piece up top)
Concave : curving inwards (as illustrated in the large pattern piece at the bottom)

Grainline Studio | Theory & Practice | Concave and Convex Curves

If you were trying to attach these two pieces together at the very edge of the pattern pieces you would want the lengths of both the convex and concave edges to measure the same amount. When sewing though, you are attaching the pattern pieces together along the seam line, a line that falls inside of the outer edge of the curve. Since the distance along the outer edge of the convex curve is longer than the seam line and the distance along the outer edge of the concave curve is shorter than the seam line, your outer edges will not match in length. Your seam lines, however, will be the same length as illustrated below.

Grainline Studio | Theory & Practice | Concave and Convex Curves

By cutting the seam allowances off you can see that both seam lines are the same length, exactly what you want when trying to sew two things together – unless you’re easing or gathering but that’s another story entirely. If you’d like to see how to sew convex and concave curves together I have more photos and text after the jump.

Grainline Studio | Theory & Practice | Concave and Convex Curves

This is what your pieces look cut in fabric.

Grainline Studio | Theory & Practice | Concave and Convex Curves

Grainline Studio | Theory & Practice | Concave and Convex Curves

Start by pinning both ends of the seam together.

Grainline Studio | Theory & Practice | Concave and Convex Curves

Grainline Studio | Theory & Practice | Concave and Convex Curves

Then pin the rest of the way around the curves. Don’t worry about the edges matching up, ease the extra fabric in, the seam lines are the same length so you don’t need to worry.

Grainline Studio | Theory & Practice | Concave and Convex Curves

Grainline Studio | Theory & Practice | Concave and Convex Curves

This is what you get when you’re done. For this pocket we’re pressing the seam downwards so we need to clip the seam allowance of the concave curve to get it to lay flat. Were we to press the seam up we would want to notch the seam allowance of the convex curve.

That’s it! Any questions just let me know in the comments below.

16 replies on “Theory & Practice | Concave and Convex Curves

  • Rochelle New

    I love tutorials like this. Sure it seems like a very basic principle, but it’s still nice to actually see it work out in practice. I’m taking a pattern making course now so I’m sure I’ll be learning all about concave/convex curves and how they have to be drafted in order to work together. Which I’m really excited about because I’m a nerd.

    p.s. I nominated you for the Craftsy Blogger Awards for best sewing tutorials because I always appreciate your posts just like this one 🙂

    Reply
  • Rachel W.

    Oh, this makes so much sense, but I’d never seen anyone put it together this way! Thank you for explaining it so simply!

    Most of my convex and concave trouble comes from sewing princess seams: I was taught that it requires staystitching and clipping and easing because ‘one edge will be longer than the other.’ Now that I think about it, that can’t be true of the stitching line… right?

    Reply
  • Annet M

    Not an issue I’ve come across yet, but probably one I’d be asking in the next while now that I seem to be ramping up my sewing outputs – so wonderfully explained!

    Reply
  • Mary M

    This is awesome. When I made my maritime muslin I was so confused at why my seams weren’t matching. For my final, I chalked my seam lines and matched those up- worked like a charm. 🙂

    Reply
  • -N-

    When I teach, I use the example of “walking into a cave” to remember “concave.” You are “vexed” if you cannot, because you will run into something that sticks out.

    Great tutorial!

    Reply
  • Jennifer in KS

    I struggle so with the 3D aspects of these types of tasks. As soon as I saw your images I immediately remembered several patterns I have made – to varying degrees of success due to this convex/concave issue. Thank you for the great visuals!

    Reply
  • Ted

    Thank you SO much for posting this. I’ve been trying to make a pattern that has a concave segment sewn to a corresponding convex segment, and was getting SO frustrated because I couldn’t wrap my head around why I was ending up with uneven edges. It never occurred to me that the parts that needed to be of equal length was within the seam allowance! Thanks for calming me down.

    Reply

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