This may be the biggest legal controversy to engulf the mathematical constant pi since that time in 1897 when the Indiana legislature tried to declare it equal to 3.2: A Brooklyn artist is claiming a broad trademark in T-shirts, jackets, caps, and other apparel featuring the Greek letter, resulting in the mass, temporary removal of thousands of products from the custom t-shirt printing site Zazzle.

At issue is U.S. trademark registration 4,473,631, issued to one Paul Ingrisano, aka “Pi Productions Corp” of New York. In January, the U.S. Patent and Trademark Office gave Ingrisano a trademark on the symbol π.—pi followed by a period—a design Ingrisano uses on T-shirts sold at some brick-and-mortar stores.

When Ingrisano discovered that California-based print-on-demand outlet Zazzle offered an array of clothing items that feature pi–which represents the ratio of the diameter of a circle to its circumference–he had attorney Ronald Millet send the company a strongly worded cease-and-desist letter this month demanding their removal.

“It has been brought to our client’s attention that your business, Zazzle Com/AKA Zazzle Inc., has been using the mathematical symbol ‘pi,’ referred to herein as the ‘PI trademark,’ in association with the marketing or sale of your products or of products offered through your services,” Millet wrote on May 16. “We have evidence of your unlawful products to preserve as evidence. Accordingly, you are hereby directed to CEASE AND DESIST ALL COPYRIGHT INFRINGEMENT.”

The letter also demanded that Zazzle provide a full accounting of all sales of products featuring pi, and open its books to an independent audit to ensure no pi profits went unaccounted for.

Rather than risk a lawsuit, Zazzle responded this week by temporarily banning all garments featuring the 3,000-year-old symbol. “This impacted thousands of products,” says Zazzle spokeswoman Diana Adair. “How many actually sold would be a much smaller number of course.”

Pi is a popular symbol in mathematical puns, so the pi-pocalypse lit a firestorm in the print-on-demand T-shirt world. “This is asinine,” wrote Dave Lartigue, whose banned “Pi plus E” shirts asserted that pi summed with the mathematical constant e add up to a tasty dessert. “Zazzle gave me instructions on how to file a counter-notice, and I plan to. Not because I really care about the dumb design, but because this is ridiculous asshattery that shouldn’t be allowed to continue.”

“This would be like McDonalds claiming the letter M as a trademark,” wrote Jez Kemp, whose Zazzle store offers apparel imagining pi dressed in a pirate costume. “The trademark is in the combination of style and symbol, not the symbol itself.”

Attorney Millet defends the cease-and-desist letter. He says that to his knowledge none of the designs sold through Zazzle included the exact trademark π.—pi followed by a period—but some of them were confusingly similar to his client’s design.

“Some clearly have a pi sign and look similar enough that folks out there might confuse it with products that my client also sells,” he says. “I saw the back and forth on the blogs of some of the sellers on Zazzle expressing their disappointment. I can see that as an understandable reaction, from a personal standpoint.”

But Stanford law professor Mark Lemley says Millet is trying to square a circle. A trademark is supposed to be used to brand a company and its products. “If you want to sell T-shirts and on the tag your brand name is the symbol pi, I think that’s a reasonable trademark,” he says. “But if what you want to do is actually prevent people from using the symbol pi on a T-shirt, then I think you’re missing the point of trademark law.”

After two days of being flooded with complaints over the takedowns, Zazzle reversed its decision on Friday, and began restoring the pi shirts to their stores. Millet says he plans to consult with his client to decide what to do next.

I asked Millet if Ingrisano has a trademark on any other mathematical constants. “No,” he says. “None that I’m aware of at least.”

Zazzle Pi Trademark Letter