These units can also be seen as units of force per unit of area (negative pressure), and the i j element of the tensor can also be interpreted as the force parallel to the i th axis suffered by a surface normal to the j th axis per unit of area. Indeed, the diagonal elements give the tension (pulling) acting on a differential area element normal to the corresponding axis. Unlike forces due to the pressure of an ideal gas, an area element in the electromagnetic field also feels a force in a direction that is not normal to the element. This shear is given by the off-diagonal elements of the stress tensor.