Thanking loyal customers for their business may seem uncontroversial, but Citigroup has something to say about how exactly you do it.

The bank sued AT&T on Friday over the telecom company's use of the trademarks "thanks" and "AT&T thanks" in a new customer-loyalty program. The use infringes several of Citigroup's trademarks, including "ThankYou" and "Citi ThankYou," according to a complaint filed in Manhattan federal court.

Citigroup, the world's biggest credit-card lender, has been using the "ThankYou" trademark in a customer-loyalty program since 2004, and more than 7 million of its customers have a ThankYou-branded credit card, the bank said. Citigroup wants a court order barring AT&T from using the terms.

AT&T's use of the phrases "is likely to cause customer confusion," the bank said. The trademark designs also have similar fonts and word placements, according to the suit.

AT&T is balking at New York-based Citigroup's allegations.

"This may come as a surprise to Citigroup, but the law does not allow one company to own the word 'thanks,'" AT&T spokesman Fletcher Cook said in an e-mail. "We're going to continue to say thanks to our customers."

Dallas-based AT&T launched the new customer loyalty program on June 2, even though the company was already aware of Citigroup's use of related trademarks, according to the complaint. The companies even have a co-branded credit card that gives "ThankYou" points to users based on how much they spend, according to the card's website.

The case is Citigroup v. AT&T, 16-cv-4333, U.S. District Court, Southern District of New York (Manhattan).

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