The sandwich is a type of food structure, originally designed to be held by a human hand and eaten without creating a mess. Sandwiches typically consist of two pieces of bread and an internal filling of meat, cheese, sauce, or vegetables. In practice, other forms of sandwiches exist, notably sandwich and lettuce wraps, hot dogs made with a single split bun, folded tacos, handheld pies, and filled buns. The general requirement is that “One thing between two things is a sandwich”, and that the existence of an exterior shell designed to be held by the human hand and an interior core of delicious fillings defines a sandwich.
Sandwiches are enormously popular around the world and are one of the most commonly consumed meals. They are typically the main part of a meal and are often accompanied by a side such as a bag of a chips, a pickle, or French fries. Most supermarkets and groceries specifically stock sliced breads, meats, and cheeses for primary use in sandwich preparation. Sandwiches are frequently made at home by people with any level of cooking skill. Restaurants, fast food establishments, and food vending machines often have a sandwich option, and with some locations it is the primary item available. Entire franchise chains base their business around their distinctive sandwich style, notably including McDonalds, Subway, and Burger King.
The sandwich was named after John Montagu, the 4th Earl of Sandwich, though the structure of putting meat between pieces of bread was invented in China around 200 BC. The general idea that fillings could be placed inside a bread is somewhat intuitive and likely was practiced globally for hundreds if not thousands of years prior to its current etymology as a sandwich.
Sandwiches increased in popularity in the United States at the end of the 19th and beginning of the 20th century corresponding with the end of the Industrial Revolution period, increased use of refrigeration, and the huge population boom that followed. Around this time, famous American historical food traditions were developed, notably the peanut butter and jelly sandwich as well as the hamburger.
The century that followed up to the modern sandwich era resulted in a localization of preferred sandwiches, with several regions adapting a variant to their local cuisine. Specific examples include Jewish Deli Sandwiches in New York, Philly Cheesesteaks in Philadelphia, Po’Boys in Louisiana, Submarine Sandwiches across the Eastern Seaboard, Buttys in the United Kingdom, Gatsbys in South Africa, Tortas in Mexico, Cuban Sandwiches in Florida, Hot Browns in Kentucky, and Italian Beef in Chicago. Each of these types has distinctive adaptions to locally popular ingredients and are generally visibly or structurally different from each other in some way.
To learn more, here are some excellent resources on sandwiches.
Food Timeline - History of Sandwiches
Serious Eats - How to Make the Best Sandwich
New York Times - Field Guide to the American Sandwich