2010 Honda Accord Crosstour EX-L (© American Honda Motor Co., Inc.)Click to enlarge picture

Although its coupelike shape somewhat limits cargo volume, the Honda Crosstour was designed with cargo hauling in mind.

Just when Americans were getting used to the term "crossover," along comes another head-scratching, lexicon-taxing auto phenomenon: the humpback. Characterized by their sloping fastback roofs, rear hatchbacks and whatchamacallit looks, these machines are further erasing the traditional distinctions not just between cars and trucks, but among sedans, coupes, hatches, wagons and even sports cars.

The mix-and-match brigade includes the Porsche Panamera, BMW X6 and 5-Series GT, Honda Accord Crosstour, Acura ZDX, the upcoming Audi A7 and the high-priced Aston Martin Rapide; i.e., seven oval-shaped pegs that don't fit neatly into any of the traditional square auto-segment holes.

View Slideshow:  Cars That Defied Categorization

Scaling Down
Consider the BMW X6 (and higher-powered X6 M offshoot) and Acura ZDX, for example. Both start with the basic bones of a midsize crossover utility vehicle — the BMW X5 and Acura MDX, respectively. But in a paean to style, designers shear and taper their roofs toward the rear, actually reducing cargo space and rear headroom compared with the SUVs that spawned them. In addition, they ride closer to the ground than their sport-ute siblings, but higher than any comparable sedan. The result might be termed a crossover coupe, but even that label doesn't describe what the surgery has wrought.

The ZDX and X6 are hot-handling and almost frivolous, with little interest in chauffeuring kids to soccer practice or making trips to Home Depot. While the BMW and Acura will hold four adults, they're being marketed mainly to empty-nesters as indulgent lifestyle vehicles, with luxury and performance taking top priority. And whatever you call them, they look and drive nothing like your typical crossover. (BMW, straining for its own definition, dubs the X6 a "Sports Activity Coupe.")

Read: BMW X6 Reviews

Scaling Up
Other models have a different philosophy: Instead of scaled-down SUVs, they're scaled-up sports cars, coupes or sedans with added cargo space. The new Porsche Panamera, for instance, was designed to evoke the sexy lines of a sports car, but in a 4-door hatch that's as long as a full-size sedan. The selling points are obvious for Porsche: The Panamera has an adult-size back seat and far more cargo space than any of its other vehicles — except the massive Cayenne SUV, of course. Even more importantly for those who love to drive but need more utility, the Panamera is nearly as thrilling to drive as a typical Porsche sports car — and far more adept than any conventional sedan on the market.

Michael Mauer, the Panamera's chief designer, says the car was born from intensive discussions at Porsche over how the automaker could expand its lineup. The company had courted controversy before in trying to expand its market offerings — and scored a worldwide sales triumph — with the mighty Cayenne. This time, the folks in Stuttgart knew they wanted a 4-seater to complement the automaker's less practical sports cars. But the company rejected the idea of a traditional sedan, preferring to push the envelope instead.

"The idea of the tailgate was born," Mauer says, along with the Panamera's coupelike roofline and proportions that could accommodate four adults and their gear.

Analysts say that, in this economy, the Panamera (which starts at just under $90,000) will likely attract buyers who have big bucks but can no longer justify splurging on a 2-seat weekend toy.

Compare: Porsche Panamera vs. Mercedes CLS-Class vs. Audi A8

Going Against the Grain
One of the most promising features of these cars is that they defy some conventional economic wisdom: that in a slumping economy, car companies would stop releasing new products in an attempt to conserve valuable resources (i.e., save money). If anything, the humpback brigade shows that many automakers actually are expanding their product lines and continuing to look for the next big thing.

What do you think about the new humpbacks?

Hence, these Frankenstein-like science projects are sprouting up all over the automotive landscape, says Jeff Schuster, director of vehicle forecasting for J.D. Power and Associates. "You don't want the company down the street to have a vehicle you don't have, and see them do well with it," Schuster says.