(NEXSTAR) – The McDonald’s Filet-O-Fish isn’t a half-sandwich. So why does it only have a half-slice of cheese?

Since becoming a permanent McDonald’s menu item in 1965, the Filet-O-Fish has been made with fried fish, tartar sauce, and — according to McDonald’s — a mere half-slice of cheese. But it wasn’t initially conceived with only a half-slice: When the sandwich was first developed by McDonald’s franchisee Lou Groen, it had no cheese at all.

Groen, who wanted to offer a fish sandwich as a way to attract customers during Lent, once told The Cincinnati Enquirer that his original recipe consisted of fried halibut and tartar sauce on a bun.

The executives at McDonald’s headquarters in Chicago didn’t love the idea at first, but relented after Groen’s sandwich massively outsold Ray Kroc’s proposed idea for a meatless sandwich — a pineapple-forward “Hula Burger.”

At the request of McDonald’s, however, Groen was forced to swap halibut for a cheaper fish — a move which prompted him to make a small addition to the sandwich.

“I had to fall back on Atlantic cod, a whitefish, and I added a slice of cheese. But my halibut sandwich far outshines that one,” he told the Cincinnati Enquirer.

It’s unclear whether Groen or the execs at McDonald’s headquarters stipulated how much cheese to put on the Filet-O-Fish, but McDonald’s maintains that the half-slice was a constant since the sandwich hit the nationwide menu in 1965, despite other small changes over the years (and a switch to Alaska pollock instead of cod).

As for why it’s only a half-slice of cheese? McDonald’s has previously provided canned responses to this question on Twitter (now X), telling critical customers that a half-slice will “save on the flavor of the sandwich.” But an older post from McDonald’s Canada sheds a bit more light.

“We only use half a slice of cheese because that’s exactly the perfect balance for the tangy tartar sauce and crispness of the Filet-o-Fish coating,” a representative for McDonald’s Canada told an inquisitive customer in 2012. “We use a whole slice on our Double Filet-o-Fish. Anything more than that would throw off the mojo of the sandwich and overshadow the delicate flavour of the pollock.”

A representative for McDonald’s U.S. did not return Nexstar’s request for additional information. But in the company’s aforementioned Twitter posts, McDonald’s U.S. made it clear they weren’t as insistent on upsetting the “mojo” of the sandwich.

“If it’s a full slice you want make sure to specify that next time you order,” the tweets concluded.