More important than silhouette. More important than the quality of the construction. The thing everyone can afford but also, if casual observation holds true, the easiest to get wrong. And that's the fit of a jacket, which should stay where it's supposed to even when it's in an awkward position like the one on Luca di Montezemolo as he points out the body curve on a new Maserati Quattroporte.
Starting from the top, the jacket collar should hug the rear of the shirt collar at all times.
The jacket shoulder should end at the edge of the shoulders, and the armholes should begin no more than an inch below the armpit. High armholes help a jacket to ride properly through a range of motion.
The position of the jacket's buttoning point should be at the natural waist or half an inch below it to keep it from bunching up when the wearer is seated.
The jacket should also be large enough to button without strain - but not too large. There should be no more than three inches of space between the button and the chest.
Jacket lapels should fall straight down the chest without buckling or pulling away from the chest in any other way and the jacket back should not have horizontal creases anywhere along its length. If a coat does buckle or crease it is usually too small, and that's not a correctible problem.
Finally, the sleeves should show half an inch of shirt cuff when the arms are hanging straight down.
A jacket doesn't have to come from Rome's Caraceni Sartoria to fit properly. Do your jackets fit?