professional bad example

oh i finally understand how a fitted sleeve actually works. it's a sort of hinge, just like the sleeve cap: in the case of the sleeve cap there's enough overarm length to fit the relaxed arm and enough underarm length to fit the extended arm, in the case of the sleeve there's enough length on the front of the sleeve to fit the relaxed elbow and enough length on the back to fit the bent elbow. if you want to be able to comfortably raise your arm above the shoulder level without causing the garment to ride up, you need a sleeve with a flatter sleeve cap and longer underarm seam, which means there's enough excess fabric under the arm when it's relaxed to form visible wrinkles... if you wanted a garment that allowed you to comfortably bend your arm as far as possible without causing the sleeve hem to ride up, you would have visible excess length on the back of the arm when it was relaxed, a lot like this. there's a lot to be learned from medieval clothing construction! the modern fitted sleeve is a compromise that will let you comfortably raise your arm to the shoulder level and bend your arm to a 90° angle and will look sleek while the arm is relaxed. and there might also be a limit to how much of an angle you can get in a one-piece fitted sleeve - the way i learned to draft it, the front seam is still a little shorter than the back after the dart has been sewn up and needs to be stretched to fit it, and a larger angle would probably eventually make the difference so large you wouldn't be able to get a good fit anymore.)

(and possibly, this means that 18th century long sleeves also have such an acute angle because it provides more freedom of movement, not because people were expected to hold their arms that way, as many seem to think)