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Atari Games created a line of NES products under the name Tengen and took a different approach. The company attempted to reverse engineer the lockout chip to develop its own "Rabbit" chip. However, Tengen also obtained a description of the lockout chip from the United States Patent and Trademark Office by falsely claiming that it was required to defend against present infringement claims in a legal case. Nintendo sued Tengen for copyright infringement, which Tengen lost as it could not prove that the legally obtained patent documents had not been used by the reverse engineering team. Tengen's antitrust claims against Nintendo were never finally decided.
Tengen got a description of Nintendo's chip from whom?
United States Patent and Trademark Office