How the Spline Shaped Some of Your Favorite Classic Cars

This story first appeared in the September/October 2024 issue of Hagerty Drivers Club magazine. Join the club to receive our award-winning magazine and enjoy insider access to automotive events, discounts, roadside assistance, and more.

The lexicon of car design includes the coolest names. An abruptly chopped tail is called a Kammback. A rear pillar with a blunted corner is known as a Hofmeister kink. Tumblehome refers to the taper of the greenhouse above the beltline. In my tenure as an automotive design student, I found that if there was a distinct feature or styling element on a car, there was usually a name for it.

But I was driven nuts when I encountered the pronounced ridge that runs the centerline of cars like the 1959 Cadillac El Dorado, 1963 Corvette Coupe, and the boat-tail 1971 Buick Riviera. The ridge was obvious to any observer, but it seemed to have no name. Wikipedia, always a great source, claims it’s called a Batsman crease because it looks like the central ridge on a cricket bat. That sounded plausible, but the only corroborating source was a random poster on a forum for industrial designers. Also, who the hell at the GM design studio in midcentury America was paying attention to cricket?

Further Googling of “Batsman crease” only led to news about cricket matches, so I reached out to Richard Vaughan, a veteran car designer who penned the Lincoln Navigator. “Most designers would call that peaked line a ‘spline’,” he said.

To anyone who has messed around with computer-aided-design (CAD) software or drawn curves with the pen tool in Adobe Creative Suite, this term should sound familiar. I asked Vaughan if the term originated from computer software. It did not: Spline was an old word, he said, carried over from the days of clay models and wooden bucks. This explanation opened up a can of worms.

The Cambridge Dictionary defines spline in two ways: as “a long, thin piece of wood or metal used in construction” and as “a curve that connects particular points.“ Humans have been using splines forever, most notably by bending wood strips to create the complex shapes of boat hulls. As boats got larger and more complex, translating the curved sections of scale models into life-size drawings and parts became increasingly difficult.

In the early days of the automobile, coachbuilders would typically line up sectioned slats of wood or metal to create a skeleton form, called a buck, around which they would shape the sheetmetal. During the forming process, the panels are repeatedly test-fitted to the buck until they conform exactly to the contours of its curved sections—splines—which define the shape of the car. As automobile production moved from small shops to assembly lines, this method of metal shaping became unwieldy. It didn’t help that the cars themselves were getting curvier, meaning the number and complexity of splines dramatically increased.

French manufacturer Citroën figured out how to define splines mathematically in 1959, which made the process of translating them to factory tooling easier and more precise. Citroën’s mathematical breakthrough was improved upon by Renault and GM, and it became the basis for the CAD software that designers use today. A glance at today’s cars proves that designers are still thinking about splines and often accentuate them by making them proud of the surface, as on the Bugatti Tourbillion (below) or most modern Cadillacs.

The next time you see a CT5-V or ogle a ’71 Riviera, tip your hat to boat builders and French mathematicians.