one question and it's variations comes up all the time so i decided i should give my answer a permanent home. the question is: why do skirt hemlines need to be curved?
it's actually a great question, one that doesn't make a lot of sense until you see it in action.
the basic skirt front sloper--the pattern from which almost all ordinary skirt designs come from--drops straight down from the hipline to the hem (click on any image to see an enlargement):
many people prefer their skirts to have a little swing to them, so they add a little extra to the side, thinking that will make the skirt more of an a-line. the problem is that if you just draw a line straight down from the waist to the hem level, you are actually lengthening the side seam, as well as changing the shape of the side seam.
original side seam:
side seam redrawn:
this might not seem like a lot of difference, but any difference will show up by making your skirt that much longer at the sides than in the front.
in order to maintain the side seam length (and shape, i should mention), you must rotate the fullness that is in the darts from the waist to the hem. this gives the skirt some movement without changing ANYTHING in the original pattern as far as seam lengths or waistline fit.
this is a very simple pattern adjustment! all you do is slash your pattern from the bottom of the dart to the hem, then close the dart, thus opening the hemline. the side of the skirt will literally swing out:
notice the red line in this drawing: if you connect the gap in the hemline, you get a CURVE. however, you haven't changed anything else--the waistline will still fit exactly the same--it is as if the dart has been pre-sewn! the side seam length is still the same, so the hem will still be level with the center front when the skirt is on your body.
if you want more swing, you just repeat this process with the second dart (notice: you will be rotated the first section again, since it is within the rotation area):
the more you rotate, the curvier the hemline gets. this process can be repeated over and over, even though both darts have been rotated. all you do is just make more slashs from the waist to the hemline, and swing the skirt hemline but LEAVE THE WAISTLINE INTACT.
you must repeat this process on the skirt back, but because the darts are different lengths from the front skirt, you will have to adjust the side seams so they balance--just average the difference between the front and back hemline lengths and adjust accordingly.
at it's most extreme, this process can be used to create an entire circle skirt, although it would be tedious and certainly would not give a better circle pattern than just drafting it from scratch.
all the images used in this post were created with the software i use to teach patternmaking at fashion design online. if you are interested in learning more flat patterning techniques, consider joining one of our classes, we teach you not only how to make patterns but also how to use CAD tools to automate the process. and frankly, it is just fun to meet up with other people how share the same passion for making 2D fabric turn into 3D objects like clothing.