Just as the definite integral of a positive function of one variable represents the area of the region between the graph of the function and the "x"-axis, the "double integral" of a positive function of two variables represents the volume of the region between the surface defined by the function and the plane that contains its domain. For example, a function in two dimensions depends on two real variables, "x" and "y", and the integral of a function "f" over the rectangle "R" given as the Cartesian product of two intervals formula_43 can be written