A loaf is a shape, usually rounded or oblong, mass of food. It may refer to a whole article of bread, or meatloaf. Technically, any unit of bread is called a "loaf", no matter what its shape, and the loaf can therefore vary in the ratio of length to width, and in its roundness. However, it is common to bake bread in a rectangular bread pan, also called a loaf pan, because different kinds of bread dough have different levels of viscosity, meaning that some doughs will tend to collapse and spread out more than others during the cooking process. Doughs with a thicker viscosity can be hand-molded into the desired loaf shape, and cooked without using any kind of walled pan. However, using a bread pan with sides higher than the height of the uncooked dough maintains the shape of doughs with a thinner viscosity, and allows multiple loaves to be cooked with different dough recipes while maintaining approximately the same shape.