MecMovies 4.0 : M12.13: Absolute Maximum Shear Stress

The state of stress shown on this element is termed plane stress.

But, what do we mean when we call this situation plane stress? After all, it is obvious that the $x$ face is a different plane surface than the $y$ face. How can we say that all of the stresses acting on this element are in the same plane?

Note that the normal to the $x$ face of the stress element (i.e., the $x$ axis) lies in the $x y$ -plane.

Likewise, the normal to the $y$ face (i.e., the $y$ axis) also lies in the $x y$ -plane.