A lot actually. When you are designing a garment, the armhole–also called an armscye–is one of the keys to a good fit. It can affect the fit through the front and back, the balance of the sleeve, and the overall comfort of the garment. And most importantly, for sewing for women, the armhole affects the fit of the bust. On the dress form, you want a balanced armhole with the shoulder seam at the top and the front and back of the armhole encircling easily that circular metal plate on the form that's called an armplate. You don't want pull or drag lines and should be able to slide a couple of fingers under the fabric at front and back.
Armholes need a lot of ease. If you think about it, it makes sense. You need the forward and backward motion of the arm and shoulder, as well as being able to lift your arms. That's why designers lower the armhole about 2 inches at the top of the side seam on woven garments. If you've ever put in a sleeve, you will notice the ease at the shoulder cap, about 1 to 1-1/2 inches, that allows the cap to be eased into the curve of the arm cap. You will notice, too, the one notch at the front and two notches at the back of the armhole. Those notches match up to the sleeve notches, and are especially helpful for industry production to determine the front and back easily–a method that's been carried over to home sewing patterns as well..
One of the most fascinating things about armholes is how they can be transformed, to a raglan or dolman sleeve, for instance. You would think by the look of it, that the designer just added some room and moved seamlines to create either, but the truth is that both the raglan and dolman are based on the fitted straight sleeve and basic bodice. Through rotating and moving darts and adding ease and style lines, you can create either one of these elegant sleeves.
As someone who was a home sewist for years, never really understanding the basics of patternmaking, it's fascinating to me to see how many styles can evolve from the basic bodices and fitted sleeve. If you have adjusted a bodice pattern to fit you well, the sky is the limit on designs you can create with the assistance of a good patternmaking book.
For an adorable summer dress based on the raglan sleeve, check out the Sunny Day Beach Dress pattern in the Sew Daily Shop.
Have you adjusted a pattern to make a new design? I would love to know what you've done.