When sewing a curved seam, you are supposed to be clipping or notching the seam allowance, so that the seam can stay flat and smooth. The rule is that when the seam is concave, you clip the seam allowance; when the seam is convex, you notch the seam allowance. This is an important step. Without clipping or notching the curved seam allowance, you end up with an ugly bulge or awful pull. Worse yet, this may involve your neckline, so the mistake is front and center.
However, quite frankly, I never remember what is a concave curve and what is a convex curve. These are not words that I throw out in my daily conversations. They mean barely more than “blah blah blah” to me. So they are not how I remember when to clip or to notch a curved seam. Besides, my memory has gone down the toilet. It used to be so good (if I remember correctly), but the good ol’ days are gone. Now I rely on logic. If it makes sense to me, then I will know what to do.
That’s why I prefer to think of this problem as allocating space for the seam allowance.
Let me jump in by showing you an example.
I am going to sew these two pieces together along the curve.
So I stack them right sides together, and sew the seam. (In this case, I am using a silk organza, so the right side and the wrong side look the same. But I want you to be able to see through the fabric, hence the silk organza.)
Let’s disregard whether this is a concave or convex curve, and let me show you two versions of how I handle this seam allowance.
In the first version, I am clipping the seam allowance.
Clipping is pretty easy. Take a snip somewhat perpendicular to the stitch line (or tangent to the stitch line, if you want to get technical). Be careful not to snip through the stitch line though. I snip both seam allowances at the same time, but for thicker fabric, you can stagger the clips.
Clip clip clip clip clip. The curvier the seam, the closer together the clips should be.
Now we turn it right side out. By doing so, the seam allowance is being flipped to the inside.
Notice what happens here through the transparent fabric? Now the sections of seam allowance are overlapping each other inside. This is not ideal because we want to reduce bulk. Having the sections overlapping each other is bulky!
So instead, let’s look at version 2. In this version, I am notching instead.
Notching is basically cutting triangle pieces out of the seam allowance. Obviously, do not cut through the stitching line.
I also do it this way. I fold the seam allowance and cut it at an angle in one snip.
Keep doing it until you notch around the entire curve. (Yes, I know they are uneven. In real world, you wouldn’t be able to see these seam allowance sections, so it’s not a big problem if you are an uneven notcher like me.)
Now, I turn the piece right side out. That means the seam allowance will now be inside.
Even without the closeup photo, you see that the seam allowance sections are not overlapping each other. But here’s the closeup anyway.
So this is essentially what happened. When you are turning the fabric right side out, you are moving the seam allowance from the outside to the inside. Notice that outside has more space (peach color), and inside has less space (lavender color). To jam a seam allowance that occupies a big space into a smaller space, you need to remove some fabric (notching) in order it to fit. It’s like moving from a 3-bedroom house to a 1-bedroom house. You’d better get rid of some of your stuff or it won’t fit. Make sense?
By the way, if you do not clip or do not notch that seam allowance, the excess fabric of the seam allowance will bulge up like an eyesore.
Now let’s look at the seam allowance of another type of curve. This one looks different.
There are two layers here, and I stitch them right sides together along the curve.
I clip the seam allowance along the curve as before.
Clip, clip, clip. Be careful not clip through the stitch line.
Now I turn the fabric right side out. You see that the clips spread apart when the seam allowance is turned to inside?
In this scenario, clipping is sufficient. There are no overlapping seam allowance sections.
The rationale is this: when turning the fabric right side out, the seam allowance again is being moved from the outside to the inside. However, in this scenario, the outside space (peach color) is smaller than the inside space (lavender color). Therefore, I do not have to remove any fabric (notch) to make it fit. But I do need to clip it, so that the seam allowance can be spread apart to occupy the now bigger space.
By the way, if you do not clip or notch, the seam allowance can’t spread open like it wants to and the curve can’t lay flat.
The concave and convex categorization gets confusing when you have one concave curve and one convex curve, as such:
(How to sew a seam like this is off topic, but if you are interested, you can read this post about how to sew a smooth princess seam, which essentially represents this scenario.)
It turns out the theory of allocating space for the seam allowance is more appropriate. Here I am pressing my seam to the right-hand side.
You can see that only one seam allowance needs to be clipped in order for the seam to stay flat. Because the other seam allowance is not turned, nothing needs to be done to it.
However, if I decide to press the seam open, the unclipped and un-notched side of the seam allowance is now wavy due to the excess fabric.
And you’d have to remove some fabric by notching the seam allowance.
So in this scenario, the concave/convex guideline doesn’t even work. However, if you look at it as allocating space for the seam allowance, then it works.
The reality is that after a while, you are not going to think about this exercise as allocating space for the seam allowance or using the concave/convex rule. You only have three options: 1) notch it, 2) clip it, and 3) do nothing. If you have excess fabric bunching up, you notch it. If you have pulls along the seam, you clip it. And if you have no problem at all and it lays flat perfectly, do nothing. As simple as that.
I also like to point out that you can also trim the seam allowance. By doing that, you may not have to clip or notch as much for the curved seam to lay flat.
Am I complicating things and confusing everyone? I’d love to hear from you.