A pie is a baked dish which is usually made of a pastry dough casing that covers or completely contains a filling of various sweet or savory ingredients.
Pies are defined by their crusts. A filled pie (also single-crust or bottom-crust), has pastry lining the baking dish, and the filling is placed on top of the pastry but left open.
Pies can be a variety of sizes, ranging from bite-size to ones designed for multiple servings.
The need for nutritious, easy-to-store, easy-to-carry, and long-lasting foods on long journeys, in particular at sea, was initially solved by taking live food along with a butcher or cook. However, this took up additional space on what were either horse-powered treks or small ships, reducing the time of travel before additional food was required. This resulted in early armies adopting the style of hunter-foraging.
The introduction of the baking of processed cereals including the creation of flour, provided a more reliable source of food. Egyptian sailors carried a flat brittle bread loaf of millet bread called dhourra cake, while the Romans had a biscuit called buccellum.