It’s time to get technical. Let’s use pictures more than words to understand the aesthetic character of an old friend.
Musicians are used to talking of notes, timbre and tone. Choreographers use dance notation. Designers have curves but unlike music and dance, designers’ raw material lacks an accepted, verbal way to describe it. Predominantly words like “expression” describe the degree to which shapes have a character. Words are given to the end-effect of the expression too but not really to the cause. Perhaps designers can talk of acceleration, crown and fullness and that almost exhausts the basic terms. So, we must turn to maths to measure curvature and at least see some quantified visual data.
These images show curvature plots of the feature lines on the DS. The curvature is represented by a vertical bar, normal to the curve, proportional to the amount of curvature at that point. The longer the bar, the more curvature there is and the reverse: a short bar or no bar means flatness. A smoothly accelerating rate of curvature is expressed by a smooth “curvature fence”.
Below is a side profile of the DS. Notice the curvature fence over the front wheel. The height of the red lines is almost constant, suggesting an arc-like curve (an arc is part of a circle which has constant curvature).
You can also see the main feature lines in relation to the overall profile. The line of the sill curves more as it heads to the rear, projecting eventually up; the roof falls but is itself also a line of constant curvature. One might think it accelerates to the rear but it doesn’t, it is that the whole line falls back. Looks can deceive.
At the back I have extracted a key shape, the trapezoid of the tail lights and license-plate surround. This shape turned up again on the XM, albeit wider and taller. The CX and XM also repeated the trope of having no wraparound of the lamps.
Out of the main lines, you get the above diagram, an instantly recognisable profile with instantly recognisable interior graphics and feature lines.
Bertoni was not thinking in terms of curvature fences when designing the DS, it was intuitive sculpture. These days every square centimetre of the car has been subject to curvature analysis to the extent that often the designer is studying the curvature of the curvature fence itself. With that looking smooth and with no unwanted sudden bumps or flats, they can be assured the resultant highlights are as smooth as body-pressing and painting can make it. Often it is the paint itself or the vagaries of metal pressing that stand in the way of the smooth highlights seen on the CAD screen.
Even if we are not so happy with the overall look of cars today, none leave the factory with intentionally duff highlights. And that was an interesting insight I had when studying the current Ford Fiesta. It has a remarkably high quality of surface refinement which lends to the car a sumptuous quality and an appearance of carefully honed sculpting. I admit I was sceptical about the Fiesta but the curvature quality has had a strong impact on my perceptions of the vehicle.
Driven To Write 
Great thought provoking piece as usual Richard. This car has always fascinated me and I have read many different articles written about it. This, however is all new and really puts words on why I like it so much. You didn’t think the SM had been as influenced by the DS as the CX, XM and C6?
Nice post, Richard. On the rare occasions when I see a DS in the metal I’m always struck by how low it is, but mainly by how voluptuous it is compared to modern cars. The plan view is interesting too. Although the windscreen is upright in profile view by modern standards in plan view it’s very curved, a trait shared with the classic Saabs.
Yes, thank you, Richard, for your usual excellent, perceptive analysis. As I read your piece, a question occurred to me, which you anticipated and answered as follows:
“Bertoni was not thinking in terms of curvature fences when designing the DS, it was intuitive sculpture.”
I imagine that, back in the 1950’s, the “science” of design was in its infancy (if it existed at all) so intuition and a good eye were critical in producing a pleasing result. I now wonder if there’s a risk that creativity is being stifled by the degree of analysis to which designs are subjected?
Ahhhh. A classic piece of Herriot – or even Bertandnairobi (for those who remember that era). Is it meant as some kind of Christmas special? Either way, I loved this article and have already read it a few times as it takes my brain time to absorb the association between the words and the pictures. I really do look at cars in a different way thanks to Richard’s articles and I learn new things with every ‘lesson’. Of course, it helps when the object of the subject is as wonderfully interesting and beautiful as the DS. I too am interested that Richard refers not at all to the SM in his commentary – but of course there are many references between the SM and the Xm, so I now have the impression that the Xm is a bit of a ‘greatest hits’ collection, albeit with a few original tracks of its own to pique the interest of the observer. I’ll probably write more later, but I wanted to express my initial appreciation of the thought and insight from Richard’s work.
Something subtle, but notice how the leading edge of the bonnet and the trailing edge of the boot lid converge to a slight point. The rear window of the Xantia (as seen from above) had the same motif.