Pie crust is the pastry shell, most often made from flour, fat (butter, shortening, or lard), and water, that holds a pie's filling. The fat is worked into the flour so it stays in small, unmelted pieces going into the oven; those pieces melt during baking and create steam pockets, which is what makes a crust flaky rather than dense. Pastry-wrapped fillings have ancient roots, but the flaky, all-butter or all-lard style crust common today developed later, over centuries of European baking. Fillings can be sweet (fruit, custard) or savory (meat, vegetables, cheese), and a pie can be single-crust, double-crust, or lattice-topped; the crust's flavor is deliberately neutral so it doesn't compete with the filling.