The fact that we buy eggs by the dozen has always seemed like one of those everyday facts of life that's not particularly worth thinking too hard about. But this week, while preparing an omelet, the question suddenly gripped me: Why are eggs sold by the dozen? Life can truly be surprising sometimes. One moment you’re making breakfast, the next you’re pondering the significance of the number 12. And then the next moment you're calling an egg farmer in upstate New York to ask her why eggs are sold by the dozen. At least, that’s what happened to me. 

The farmer was Betsy Babcock, co-founder of Handsome Brook Farm. “My understanding is that back in the 1400s and 1500s in England, eggs were sold in dozens because there were 12 pence to a shilling, so you could buy 12 eggs for a shilling. So when they came over from England to the States, they brought that concept over here,” Babcock told me. 

Interesting, I thought. Where had she heard that? 

“That’s a piece of trivia I picked up years ago. I don’t know if it was during Trivial Pursuit or something like that. We have a lot of British friends who live near us. I asked them today to see if they’d heard that, and they’d heard the same thing,” she said. 

Turns out, a lot of people are familiar with that explanation, including Jesse Laflamme, chief executive of Pete & Gerry’s Organic Eggs in New Hampshire. “I think I may have heard it from my grandfather early on,” Laflamme said. Shauna Aeschlimann, of Oliver’s Organic Eggs, knew it, too. “I don’t know if it’s an old wives’ tale. That’s just what I’ve heard in passing,” she said.

Historical knowledge is funny, isn’t it? We know, for instance, that ice age Britons used the skulls of the dead as cups, but it’s really hard to say for sure why we buy eggs 12 at a time. I asked Dr. Monica Gisolfi, the author of The Take Over: Chicken Farming and the Roots of American Agribusiness, 1914-2007about all this, and she essentially answered with a shrug.

“I don't actually know the answer to that question,” she said. “In my own research on the poultry industry I never came across any reasoning for or against selling eggs by the dozen.”

Doug Hurt, head of the history department at Purdue University, told me that “farm women sold eggs by the dozen at least since the late 19th century,” but didn’t know precisely why. “Country merchants probably had something to do with it for shipment and sales to urban markets,” he said. 

Twelve isn’t just an important number in the food world, of course. Ancient Egyptians used a duodecimal numeral system—one that uses 12 as a base—according to metrologist Michael A. Lombardi of the National Institute of Standards and Technology, because there are 12 finger joints on a hand and 12 lunar cycles in a year. 

Bon Appétit’s Sam Dean writes that “as great as it would be to say that we have twelve eggs in a carton because we have twelve full moons in a year (the symbolism!), it probably has more to do with easy math than anything. The number 10 can only be divided by 2 and 5, while 12 can be seen as 2 x 6 or as 4 x 3, which makes it ideal for things like shipping and selling goods.” 

In some parts of the world, however, eggs aren’t sold by the dozen. They can be bought by 10s, or 8s, or by the piece. Laflamme said there’s technically nothing preventing American producers from selling eggs this way, but “everything in our entire supply chain is adapted for that one dozen” and altering it would “definitely be a retooling.” Besides, he said, Americans have gotten used to the current system of dozens, half-dozens, and other multiples of six, and they probably wouldn’t take too kindly to a change.

“It kind of makes me think of when the US tried to switch to the metric system and it was rejected,” he said. “Consumers and egg producers would probably reject anything but the dozen.”