“It’s a thing,” I told my husband, when we opened the second holiday card featuring a photo of an extended family, all sporting red and black checked pajamas. “It’s buffalo check.”

He looked confused. “Buffalo check,” I repeated. “It’s all the rage with grandparents this year. They require their adult children and grandchildren to dress in it for the annual holiday card photo.”

He wondered aloud if this was something all grandparents do these days.

“Maybe not all, but a lot of them,” I said. I gestured to the two photographs in our mounting stack of holiday cards. Each featured large extended families supporting this year’s popular uniform.

Curious, I did a little research, consulting the official Woolrich company site, as well as various history, design and fashion sources. Here’s what I learned: John Rich II, an immigrant to the United States from England, and a partner established his woolen mill in 1830. He often traveled to lumber camps and farms in the mountains of central Pennsylvania selling woolen cloth and yarns from a mule-drawn cart. In 1845, he built a woolen mill near Chatham Run in Pine Creek Township. Many of Rich’s early customers worked in the lumber industry in the area, and they were good consumers of socks and clothing from Rich’s mills.

During the Civil War, Woolrich supplied the Union army with blankets, and today, the company continues to manufacture blankets and fabric in its Pennsylvania factory. By the turn of the century, more and more people began to enjoy outdoor leisure activities. Woolrich responded to this burgeoning market, turning out wool shirts, jackets and caps. The Woolrich factory was destroyed by fire in 1901, but was immediately rebuilt, and by 1917 was manufacturing blankets for American armed services in World War I. The company introduced a buffalo check shirt in 1950.

But where did this distinctive large black and red check pattern originate? There are a few competing theories. On its company website, Woolrich says the designer of the iconic plaid owned a herd of buffalo, and thus used the name for his brainchild. But the design could have had its roots in the 18th century. A popular fabric design featuring checks — large squares of equal size in all directions, sort of a giant version of gingham — was a favorite of Queen Charlotte of Mecklenburg-Strelitz, the wife of King George III — yes, Hamilton aficionados, that King George — according to an article on HGTV.com. The queen liked the pattern so much she had chairs, sofas and bedcovers done up using the large checked material, and soon it was known as Queen Charlotte’s Check — a far cry from the woodsy, down-home moniker of buffalo check.

And some claim that the red-and-black plaid was the Rob Roy tartan, dating back to Clan MacGregor in Scotland. The pattern may have made its way to America by way of Jock McCluskey, a Scottish immigrant to Montana. This would explain why native Americans adopted the tartan and wore it into battle for good luck.

The Woolrich company not only survived during the Great Depression, it thrived. In 1939, its products — over a thousand of them — traveled to the Arctic with Admiral Byrd on three separate expeditions. And when the United States entered World War II, Woolrich again profited from U.S. government contracts for blankets, stockings and coats. The company partnered with WP Lavori in Corso, Italy, in 1998, went global in 2016, and is now known as Woolrich International, after fusing its American and European companies. At its helm today is Nicholas Brayton, a seventh-generation descendant of John Rich. That’s an impressive mercantile legacy.

So when you deck your grandchildren out in those red-and-black buffalo check pajamas for the annual holiday photo or order buffalo check outfits for your big family reunion this summer, you can take advantage of your grandparent status by telling them the story of John Rich II, entrepreneur, or if you prefer — capitalist, war profiteer and merchant chief.

Lynne Spigelmire Viti (lviti@wellesley.edu ) is a lecturer emerita in the writing program at Wellesley College and the author of “Baltimore Girls” and the forthcoming “Dancing at Lake Montebello.”