The subject of this article is the complementarity principle (CP) and the formation of its concept. The CP is represented by a set of criteria reflecting the correlation between the spheres of an individual’s life and their language proficiency; the CP illustrates the distribution of languages among the spheres of life of a bilingual or multilingual individual. Despite being an illustrative tool for studying both individual linguistic personalities and linguistic communities, the CP has its imperfections – it is obvious that criteria are unable to reflect all the nuances of reality, which should be taken into account in the analysis. The brief description of the linguistic personality of the Lithuanian-French art historian Jurgis Baltrušaitis is presented in this article as an example of systematizing data “on the basis” of the CP and its components.