Which is to say, trademark infringement.

Adidas is taking Marc Jacobs to task for selling sweaters with four stripes running down the arms, a design it says is too similar to its well-known three stripe motif. In a complaint filed with the U.S. District Court in Oregon, Adidas accused Marc Jacobs of trademark infringement and dilution, unfair competition and deceptive trade practices.

The sweaters at hand are actually Katie Hillier and Luella Bartley's designs for Marc by Marc Jacobs, not the main line, and though they do have four stripes rather than three, it's easy to see what Adidas is mad about. 

Adidas claims that its stripes have reached icon status to the public, citing a whole host of articles calling them "legendary, "ubiquitous," "signature," and, indeed, "trademark." The point there is that Marc Jacobs could be misleading consumers into thinking that they're actually looking at Adidas apparel, which could tarnish the reputation it's built among customers.

"This is particularly damaging with respect to those people who perceive a defect or lack of quality in Marc Jacobs’s products," Adidas writes.

A big part of Adidas's argument is that it's very much competitive with Marc by Marc Jacobs, thanks to its ongoing collaboration with Yohji Yamamoto on the higher-end Y-3 line. (Both brands, for instance, are currently sold at Saks Fifth Avenue.) So Adidas has more to lose from Marc by Marc using similar stripes than it would from a brand reaching an entirely different customer base doing so. 

A rep for Marc Jacobs declined to comment on pending litigation for this story. Our big question? How the fact that Marc by Marc Jacobs is being discontinued will play into the suit.