As we take notice of how the collar fits around the neck, we develop an eye for fine tailoring.
Here are some contrasting examples of the bad and the good:
Collar gap with classic V-Tug with collapsing fabric and a curved (instead of straight) left lapel.
And now for the good:
PG Director Greg Jacomet in Cifonelli (who worked with an uneven
shoulder). Here there is no collar-gap, around 2 cm of shirt collar
showing in back, a straight lapel angle, and the correct amount of front
Stefan Bernard in a Zegna jacket. Notice the close collar fit on both
sides of the neck, and the correct front panel tugging. The lapel angle
is intentionally curved instead of straight, with both lapels curved and
Pal Zileri. A nice RTW specimen on all counts.
There are a few things you can do to improve the situation of dealing
with a collar gap, ranging from wearing wide-spread shirt collars to
mitigate the appearance of the collar opening to looking at having a
tailor build up a weak shoulder on the coat, to making a subtle shift in
button placement to improve a pull of the coat to the left or to the
right (again, usually indicted by uneven shoulders). But, of course,
having the collar correctly made to form to your neck from the beginning
will save a lot of trouble in the end.