They must be plane, i.e., the coordinates of the element nodes must be in one flat plane, the xy plane of the element. They must be thin, i.e., the thickness t must be small in relation to the dimensions b in the plane of the element. Loading F must act in the plane of the element.
Plane stress elements are characterized by the fact that the stress components perpendicular to the face are zero: 
= 0 . These elements may only be applied if there is no bending outside the plane of the structure, like in walls, deep beams and the like.
