Photoelastic stress patterns of French curves, viewed using polarised light. The plastic the French curves are made from is displaying coloured interference fringe patterns. Areas of similar colour correspond to regions of similar stress. From these patterns, the direction and magnitude of stresses placed on the French curves can be calculated. During photoelastic studies, these calculations can be related to stresses in real designs by using plastic, scaled-down models. French curves are templates with edges composed of several different curves. These templates are used in drafting to draw smooth curves of nearly any curvature in mechanical drawings.