The paper describes a simple experimental technique (Fig. 1) for determination of the most decisive part of the failure envelope of masonry plates, subjected to plane stresses of opposite -sign ( heterosemous ), i.e. compression and transversal tension (Fig. 2). The need for pragmatic data on the subject is apparent: Several loading situations lead to these heterosemous conditions which may result in drastic reductions of compressive (or tensile) strengths. An additional parameter entering the delta₁, delta₂ interplay has to do with the slope of the principal stress direction versus masonry layers. This fact is treated separately in the paper.