A sensitive, simple and quick method of checking the relative expansion of glass-glass, glass-metal or glass-ceramic combinations is to measure the stress birefringence, i.e. the double refraction, in standard-shaped test seals of the materials concerned against a suitable standard glass. The large-scale adoption of this method instead of the still commonly used but less satisfactory dilatometer tests calls for close cooperation between the relevant industries. International cooperation is gradually increasing in this field. The article gives a description of the stress-birefringence method in comparison with the dilatometric method, and discusses the graphic determination of expansion tolerances for glass and metal in a given seal. It concludes with an example of the application of stress-birefringence measurements in tracing and eliminating the causes of fracture in a particular product.