It follows that in the quest to achieve full and proportional coherence in your outfit every aspect has to be considered. It goes without saying then that your jacket sleeve is one such aspect. Just as a shirt can be altered and made slimmer, a jacket should enjoy this benefit as well. Jacket sleeves cannot only be lengthened or shortened, they can also be tapered. The above two images clearly evince this. The light grey jacket, at top, has undergone so many changes that the sleeve has been left hanging with a very loose fit. There has to be a consistency in the whole sleeve therefore, from the sleevehead to the opening. The tapered or not tapered sleeve can also look two ways to a viewer. The loose, baggy sleeve can give the effect of two different garments at the wrist, especially when the shirt is slim and the jacket is baggy. It's even worse when both the shirt and jacket sleeve are baggy. The top image illustrates the difference between a slim shirt and loose jacket sleeve.
A tapered sleeve therefore shows consistency, tidiness and is pleasurable to one's eyes. When one is standing with their arms at the sides there is no looseness that gives the illusion of the shirt being lost in the jacket sleeve. The rule of showing showing shirt cuff becomes very applicable because proportion then becomes very amplified.
PG: Man to man, generation to generation.