From elasticity theory, an infinitesimal volume of material at an arbitrary point on or inside the solid body can be rotated such that only normal stresses remain and all shear stresses are zero. The three normal stresses that remain are called the principal stresses:
| σ1 - Maximum |
| σ2 - Middle |
| σ3 - Minimum |
The principal stresses are always ordered such that σ1 > σ2 > σ3.
