We will consider a very simple object, an axially-loaded prismatic bar, to investigate the nature of stress. The load $\mathrm{P}$ acts through the centroid of the bar.

While we may be tempted to think that there is only one stress in a material (particularly in a simple axial member), this study will demonstrate that there are many different combinations of normal and shear stress in a material. The magnitude and direction of the normal and shear stresses at any point in a solid material depends upon the orientation we choose.