While we have shown shear stress acting along the edge of a flat stress element, we must keep in mind that the shear stress actually exists on a surface. Furthermore, shear stress is always present on mutually perpendicular surfaces. Shear stress on a cross-sectional surface is always accompanied by an equal magnitude shear stress acting on a longitudinal surface.
Customarily, a point of interest in a component is symbolized by a flat square, and arrows are drawn on the square to represent the stresses acting at that point. It is understood that the square has a thickness dimension associated with it.