What's in a roof? Well, if history provides any sort of clue, a smooth shape means speed and luxury, no matter how many doors it has.

New offerings from Porsche, Audi, BMW, Jaguar, Mercedes-Benz, and even Honda, haven't invented a new class so much as joined a long list of automakers that have sought on and off since the early 1940s to distinguish themselves with a roofline style known as the "fastback".

This design ideal will be familiar to anyone with even a passing interest in automobiles; think back to the 1970s and the iconic Ford Mustang and Torino, the first Dodge Charger, Jaguar E-Type and Datsun 240Z, in which rooflines plunged dramatically in an extended sweep from the roof to the rear end, leaving no trace of a trunk compartment hanging ungainly behind the rear wheels.

Other more modern, iconic examples include the Porsche 356 and 911, Toyota Celica and the Aston Martin DB5, made wildly famous by the passenger-ejecting, oil-slick-spreading, tire-shredding sports car featured in the classic James Bond movie "Goldfinger". Many might tend to think of fastbacks as rakish two-door coupes, though numerous four-door sedans have received the treatment over the years, including the Oldsmobile Cutlass Salon and Buick Century of the early 1980s that some say clumsily brought the design back into fashion at that time.

According to the Beaulieu Encyclopedia of the Automobile, the design concept actually stems from the early 1930s when automobile designs that were ahead of their time included "teardrop streamlining at the rear" and which would, 25 years later, come to be called fastback. American fastbacks produced by the then Big Three automakers Chrysler, Ford and General Motors were known early on as "torpedo backs."

Merriam-Webster first recognized "fastback" as a definition in 1954, many years before the term "hatchback" was popularized and entered the dictionary in 1970. Depending on to whom you speak, the terms fastback and hatchback are either complimentary or mutually exclusive; there seems to be little gray area on the topic.

The Road & Track Illustrated Automotive Dictionary defines fastback as, "a closed body style, usually a coupe but sometimes a sedan, with a roof sloped gradually in a unbroken line from the windshield to the rear edge of the car. "A fastback naturally lends itself to a hatchback configuration and many have it, but not all hatchbacks are fastbacks and vice versa."

In fact, the 1963 Corvette coupe has neither a truck or hatchback. From outrageously designed finned fastbacks by Czechoslovakian manufacturer Tatra through the 1930s, '40s and '50s to one of the arguably most beautiful cars on the road today, the four-door Mercedes-Benz CLS, the fastback treatment has graced some of the more memorable automotive designs to ever roll down a highway.

Suggestive of speed and excitement, fastbacks offer an advantage in designing cars with slippery aerodynamic qualities that cut the air cleaner than other, more boxy designs. Superior aerodynamics can help reduce fuel consumption and rolling resistance.

Audi design chief Stefan Sielaff characterizes the A7 Sportback as having "an almost monolithic clarity and a vast reduction in the number of lines" in the vehicle's overall look. Translation: it's a clean design. "The roof, shoulder and sill lines appear to have been drawn with a single stroke," he adds.

A variety of four-door vehicles ranging from sports cars to tall wagons are getting the fastback treatment, with Germany-based manufacturers taking the lead. BMW's X6 and 5 Series Gran Turismo offer two different takes on the design, while Porsche's polarizing Panamera designed to fit former chief executive Wendelin Wiedeking's six-foot, 2-inch-tall frame, with lots of room still in the back seats offers a rounder, more dramatic version of the sweptback shape.

The Panamera is shaped "the way it needs to be," Porsche chief designer Michael Mauer told the New York Times. "As with all Porsche design, it begins with the function. As a four seater (and Porsche's first-ever four-door sedan), the car has to have a different side view," he said. "It has to express the package and volume, but also be sporty and dynamic."

Jaguar's newest fastback flagship sedan, the XJ, offers even more rear seat headroom 38.6 inches compared to 38.2 in the Porsche despite appearing smaller with an even more steeply raked back end. Honda's much smaller Crosstour blends "sporty, low-profile contours" with the functionality of a station wagon, and offers very nearly as much rear seat headroom (37.5 inches) as the much larger Porsche and Jaguar. Whether you're in the market for a ride that's visually exciting at the high end of the scale, or something more widely affordable, the fastback could be an idea whose time has come, once again.