Relativity theory is useful for understanding the phenomenology of the magnetoelectric effect of the antiferromagnet chromium sesquioxide Cr2O3 in two respects: (i) one gets a clear idea about the physical dimensions of the electromagnetic quantities involved, in particular about the dimensions of the magnetoelectric moduli that we suggest to tabulate in future as dimensionless relative quantities; (ii) one can recognize and extract a temperature dependent, 4-dimensional pseudoscalar from the data of magnetoelectric experiments with Cr2O3. This pseudoscalar piece of Cr2O3 is odd under time reflections and parity transformations and is structurally related (“isomorphic”) to the gyrator of electric network theory, the axion of particle physics, and the perfect electromagnetic conductor of electrical engineering.