The perfectly square loaf of bread used for sandwiches is known as a 'Pain de Mie' or 'Pullman Loaf.' It was developed in Europe using long, lidded pans that prevented the bread from rising upward, forcing it into a rectangular shape with a very thin crust and fine, even crumb. The name 'Pullman' comes from the Pullman Railway Company in the United States, which used these pans to bake bread in the cramped kitchens of train cars. The square shape was a triumph of efficiency, as it allowed more loaves to be baked at once in the same oven space and made the bread much easier to slice and stack for sandwiches. This 'everyday' shape became the standard for the industrial bread industry in the 20th century.