You know how sometimes when you clip and notch a curve, then turn it right side out it can create a very slight angle at the point where the clip or notch is? To avoid that and get super smooth curves trim and grade the seam close to the stitching line as I’ve done above. The reason this works : if you were to measure it, the length of the outer edge of the seam allowance is much longer than the stitching line. By notching you are reducing the length and allowing the seam allowance to lie smoothly inside the stitching line. Trimming close to the seam allowance effectively does the same thing by reducing the measurement of the outside edge but without creating those weak points in your curve that are prone to denting. I know this might sound crazy with all the importance put on clipping and notching as the way to do curves but if you have a ready to wear peter pan collar in your closet, check it out, it’s most likely done this way. All mine are!