Answer: Meat

In the broadest sense, a sandwich is simply two slices of bread with just about anything edible in between them. Peanut butter and jelly, ham and cheese, even sliced vegetables with some sort of condiment; all of these things would be seen as sandwiches in the vernacular sense.

With that in mind, it might be, initially at least, a bit surprising to find out that the United States Department of Agriculture has a significantly stricter definition. If you want your product to qualify as a sandwich by their book (and it’s literally by the Food Standards and Labeling Policy Book), then a closed-face sandwich must contain at least 35 percent cooked meat and no more than 50 percent bread; an open-faced sandwich must contain at least 50 percent cooked meat.

The reason there are no rules regarding things like cheese, types of seasoning, or any talk of sandwiches other than the meat-packed variety isn’t because the USDA has a vendetta against the humble grilled cheese sandwich or considers a kale and pesto sandwich to be un-American. The reason behind the USDA’s definition lies in their mission in food labeling. The USDA is only responsible for the labeling of meat, poultry, and egg products, and their Labeling Policy Book reflects that—there’s no need to define the terms for items you’re not in charge of labeling.

So who is in charge of regulating terms like “grilled cheese” to ensure that what you get is cheese and not a waxy substitute scraped off a factory floor? That would be the Food and Drug Administration—see Title 21, Chapter I, Subchapter B, Part 133 for details on what makes cheese, well, cheese in the eyes of the law. There are 12 sections alone devoted to pasteurized cheese, so rest easy knowing that they’ve hammered out all the finer points of what constitutes cheese of all sorts.