Earlier this year at the Detroit auto show, Nissan unveiled its Sport Sedan Concept, a car that a number of observers thought might be a preview of a future Nissan Maxima. Nissan's web site has it here and Automobile Magazine mentioned it here.
That was in January. Robert Cumberford, the magazine's longtime styling critic devoted a column to the Sport Sedan Concept in the magazine's May 2014 printed issue. Among his observations was this: "I hope the 'floating roof' remains [in a production version], as it greatly helps lighten the overall aspect of what is, after all, an overly thick and heavy-looking lower body." Elsewhere in the column he calls the thin, black gap between the C-pillar and the lower body a "Very nice workout." I agree.
What was Cumberford talking about? We'll just have to take a look.
A three-quarter rear view. The gap shows more clearly here; this was the image Cumberford used in his column when discussing the feature.
Transparent roofs have been styling fetish on dream cars for many years, and the Sport Sedan Concept continues the practice. This view shows how the black separation band continues around what is the backlight (rear window).
In 1953, Packard introduced its Balboa show car (see here, scroll down). It was based on Packard's Caribbean high-end convertible, with a special top crafted to create a two-door "hardtop convertible" (a body style popular at the time). The truly non-production Packard details have to do with the rear of the top. Rather than a conventional backlight, the Balboa has a flat, slightly reverse-leaning rear window. This feature eventually appeared on American production cars such as the 1957 Mercury Turnpike Cruiser.
Now notice that there is a grooved, rectangular block towards the bottom of the C-pillar. As with the black line on the Nissan, this serves to more strongly separate the roof from the lower body. So far as I'm concerned, this detail was not really necessary because 1953 Packards had lower-body "hips" that, when juxtaposed by a top that was inset and had flat widow glass on its sides, already defined visually distinct elements.