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.
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)
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.
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.
This is what your pieces look cut in fabric.
Start by pinning both ends of the seam together.
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.
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.