The fundamental advantage of two-dimensional photoelasticity is its great simplicity. Neither the frozen-stress three-dimensional method nor the photoelastic layer technique share this simplicity as yet. Whereas the frozen-stress and layer techniques are most suitable for final investigation into the strength of a finished design, the simple plane photoelasticity can be used as a tool by the designer at every stage of his design, as a specialised slide rule, so to say, that gives sometimes faster and frequently more accurate answers than complicated stress calculations. The consequences of given changes in the outlines of a component can often be studied more conveniently and cheaply by this method than by almost any other.