I have one suit jacket in particular on which the seam is significantly angled. I really love the look. All of Suchet's jackets in Poirot also have this feature.
Alden's explanation of this feature is the following:
If you will take your coat, a measuring tape and sit down in your
chair a minute I will try and explain the phenomenon for you. Place
the shoulder of the coat on your leg such that your knee takes the
place of your shoulder. You should see that the shoulder seam is
Now with your measuring tape, at the collar take the measure between
the point where the back center seam intersects with the collar and
the point where the beginning of the shoulder seam intersects with
the collar. This measurement, called the "back/neck" or "pupilla" in
Italian is the single most important one in a natural shouldered
coat (or any coat) , for reasons I will endeavor to explain later.
It should be around 3 inches (for a size 42 coat, for example.) Here
is where I need your imagination a minute. Can you see how the
position and angle of the shoulder seam would change as a function
of this measurement? The back center seam's position is fixed. If
the beginning of the shoulder seam is 5 inches away instead of 3,
can you see that the shoulder seam (always running straight) would
be considerably more forward on the shoulder. If the "pupilla" were
4 inches, can you see the shoulder seam moving back? Now at three
inches can you see that it would be well back from center? So, in
reality, the shoulder seam is straight and it position changes
relative to its initial starting point at the collar, that is a
function of the cut of the "pupilla."
A natural shouldered drape coat or any coat for that matter is
suspended from the collar and shoulders. Think of your body as being
a large hanger. The fulcrum point of the coat is the collar. The
size of the "pupilla" regulates the proper amount of tension such
that the coat is held in place at the collar. If the "pupilla" is
too long there will not be sufficient tension to support the coat
and it will shift all over the place (some of you may recognize this
phenomenon.) If it is too short other problems will occur( but
rarely is it ever too short.) The point is that in a bespoke coat,
the "pupilla" measurement can be and should be perfect. (The same
cannot be said for RTW, though some high end RTW does pay attention
to this detail despite the added expense of manufacturing.) It
should be short enough to provide the right amount of tension at the
fulcrum point of the coat to keep it glued to your neck, and draape
from that point. Since the proper measure to achieve this is shorter
rather than longer, we will expect to see the shoulder seam (in an
optical illusion) canter towards the back of the shoulder as opposed
to moving down the center.
You can imagine that in a natural shouldered coat the "pupilla"
measurement is crucial since there is no artificial padding, bale of
hay etc. , to rely upon to correct errors. If the cut is wrong, all
is wrong. There is no way to glue it together with padding.
Understand also that the "pupilla" must enjoy the company of a
perfectly cut shoulder and high armholes to complete the proper fit
in a natural shouldered coat. If this noble triumvirate is cut well
the jacket will be astoundingly comfortable and exude elegance. If
it is cut poorly, it is dust bin material. That's why cutting a
natural shouldered coat is so difficult to do well. And why so few
tailors do it well or at all. Its a "critical" cut.
In the golden age of the 1930s, there were many excellent tailors
capable of realizing this cut and the "natural" shouldered coat was
at the summit of its appeal and desirability. That is why we see the
shoulder seam position described above in the photos from that
period. Today, most tailors are content to make crude facsimiles
and few take the time or have the know how to do it properly.