Collar gap and related fit issues

[Mathematicus]

I got the idea of starting this thread from the dreaded collar gap issue I've found in a couple of my suit coats. Of course it would be more interesting to talk about all of those other "difficult" fit issues like shoulder divots, collar roll that are rarely mentioned but sometimes can happen even in vintage garments. And, naturally, it would be appreciated to know if someone managed to fix these problems.

I know that most people won't ever notice a collar gap, even if persisting in a neutral position. But I think that this is ad issue that once you note it, you'll end looking for it almost every time. I have a very nice 1930s suit with this problem, and I believe that is caused by the slighty sloped shape of the suit shoulders. It's a shame because I feel the coat tends to move away from the neck, in particular when I move.

In another jacket I have this issue, but in a very slight way, only when I raise both arms forward. In this case I believe that shoulders shape is spot on, but probably the collar of the coat is a bit large for my neck, causing the lack of tension needed to keep it in place.

Do someone can explain precisely what can be the reasons of this annoying imperfection? And has anyone experienced a way to fix it?

For those who are not familiar with the term "collar gap", here's a picture of the awful situation:

The jacket collar doesn't sit at the wearer's neck properly, giving the impression that the coat is "flowing" over the shoulders.

 

[herringbonekid]

if you have a recurring problem with collar gap, then chances are you have a slightly forward-leaning-posture, which means that any 'regular' cut jacket/collar will be inclining slightly away from the back of the neck.

conversely, a very erect posture will push a 'regular' cut jacket/collar back slightly, creating horizontal folds beneath the collar at the top of the jacket.

 

[Mathematicus]

Yes, I think I have a slight forward-leaning posture but I have some coats whose collars sit very well and stay glued even in extreme situations and they are cut regularly. It's not so simple to explain, in my opinion.

But is there a solution?

 

[herringbonekid]

But is there a solution?

there is no easy solution, no.

in tailoring terms it's basically a 'low neckline' problem; the under-collar would be taken off and moved higher to hug the back of the neck.
other factors could also contribute, e.g. low armholes, tightness across the shoulder blades, or front - back 'balance'.