Every serious tailoring house answers the same question first, and answers it differently: how much of the jacket should the canvas do, and how much should the body do?
Naples answers with almost nothing. The Neapolitan shoulder is built with the lightest possible canvas — sometimes none at all in the sleeve head — and the sleeve is set in like a shirt sleeve, gathered slightly at the top into what tailors call the spalla camicia. The result moves with the wearer instead of holding a shape independent of him. It is the least formal way to build a serious jacket, and for warm climates and long days, the most honest.
London answers with structure. A proper Savile Row shoulder carries real canvas, often a roped sleeve head that stands slightly proud of the shoulder line, and enough internal architecture that the jacket holds its silhouette whether or not the man inside it is standing up straight. This is not vanity — it is why the cut reads as authority from across a room. A soft-shouldered jacket photographs differently depending on posture; a structured one does not.
New York took both and asked for less. The American sack coat — looser, straighter, a single vent, a natural shoulder with just enough canvas to hold a clean line — was built for men who wanted to look competent, not aristocratic. It is the quietest of the three cuts, and that quietness has always been the point.
None of these is more correct than the others. They are three different answers to what a jacket is for. The right one depends on what you actually want a suit to do for you — and that is the first question we ask, before fabric, before buttons, before anything else.