The Fig. 4.36 shows a body of area A. Consider a small area dA. The moment of the area about x-axis is y.dA. Now the moment of y.dA about y-axis is xydA. Then xydA is known as the product of inertia of area dA with respect to x-axis and y-axis. The integral is known as the product of inertia of area A with respect to x and y axes. This product of inertia is represented by Ixy.