A lesson in patents, courtesy of a Real Housewives star and the creator of Spanx

A legal battle between Heather Thomson, CEO of shapewear brand Yummie Tummie and co-star of The Real Housewives of New York City, and Sara Blakely, creator of Spanx and personal savior of women the world over, is raising a lot of questions about design patents — especially in the fashion world.

In January, Real Housewife Thomson sent Blakely a cease-and-desist letter, demanding that the Spanx creator discontinue three slimming tank tops that allegedly copy Yummie Tummie's patented three-panel design. Blakely, who Forbes just crowned America's youngest self-made billionaire, hit back, contending that her tanks don't violate any patents and asking a judge to rule on the matter. Thomson retaliated this month with a patent infringement complaint, telling Women's Wear Daily: "I hope [Blakely is] ready for war, because I will not lie down."

Design patents in the fashion world are tricky. For instance, compare this tank top battle to last year's highly publicized Apple vs. Samsung patent fight, which ultimately sent a $1 billion message to mobile brands that making phones that look like iPhones will cost them heavily. But Apple only releases one iPhone at a time, and the shape evolves gradually (if at all) from one installment to the next. Conversely, fashion brands release a slew of products several times a year (now more than ever as the season cycle accelerates with pre-fall and pre-spring). The whole industry relies on rapidly changing consumer taste, so chasing after competitors with intellectual property squabbles can be both expensive and a monumental waste of time.

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Instead, luxury brands focus their energy on trademark infringement, gunning for knock-off vendors who mimic high-end brands' logos (which don't evolve so quickly), while diverting their eyes from legitimate fast fashion outlets like H&M and Urban Outfitters who arguably rip off designs just as readily and completely as their Canal Street counterparts.

Brands like Spanx and Yummie Tummie, however, are different. These are undergarments, after all, so designs don't have to change with the season — the products are hidden beneath clothing. That means these products have both longevity and earning power. Spanx generates $225 million to $240 million in wholesale revenue a year. Yummie Tummie won't release its numbers, but Women's Wear Daily estimates retail sales around $60 million. In that sense, it's vital that these brands protect their design patents.

According to Forbes, these suits may start cropping up with greater frequency. In December, President Obama signed the Hague Agreement to speed up patent litigation, particularly for design patents. Brands looking to shut down a copycat won't have to wait a full year anymore — during which the offending party was allowed to operate — between filing the claim and receiving the ruling. That will likely encourage more litigation.

"This is high margin stuff," patent lawyer Michael Lasky tells Forbes. "You don't want it to be ripped off. It's called waking up to reality. So few in the fashion industry realize the tools they have."