If z = f(x,y) is a function of two variables then the total differential is $\text{d}z=\frac{\partial z}{\partial x}\text{d}x+\frac{\partial z}{\partial y}\text{d}y$. Then Pdx + Qdy is said to be an exact differential if it is the total differential of some function f. Further, Pdx + Qdy is exact if and only if ∂P/∂y = ∂Q/∂x. Compare total differential.