where ∇"P" is the gradient of "P" and the second part of the equation is minus the gradient for a function of the Cartesian coordinates "x", "y", "z". In some cases, mathematicians may use a positive sign in front of the gradient to define the potential. Because of this definition of "P" in terms of the gradient, the direction of F at any point is the direction of the steepest decrease of "P" at that point, its magnitude is the rate of that decrease per unit length.