Over the years, and many mistakes, I realized that I wasn't always wearing perfect jackets. But being often in different tailor's shops perhaps I have understood and learned something. Certainly these photos speak for themselves.
The color photos are of one of my ready-to-wear suits, while the black and white photos are of my bespoke blazer, which I think is almost perfect.
The bespoke jacket should always sit as still as possible despite our body movements.
The left photo shows a non-bespoke blazer going up as I lift my arm. This is one of those things that no jacket should feature.
In fact, the photo on the right speaks for itself: my arm rises and the jacket remains in place, steady, without disruption; this is because it has a very tight sleeve lap that allows much more movement and freedom than the photo on the left.
The jacket should never detach from the neck. And should not come off in any situation you are, from when you are standing and when, above all, you are sitting, especially if the jacket you are wearing was made by a tailor.
The hips are also very important. A correct flattening and a good sartorial workmanship, with a fabric that really follows our body, allow not to have unpleasant anti-aesthetic side creases, like those you see in the picture on the left.
So, these are the three main defects that a jacket should never have, there would be others, but those, however, we send them to the next episode.