Abstract: The article presents the methodology of research in the field of descriptive diachronic spelling. The aim of the work is to develop a system of criteria for choosing spelling variants at different stages of the formation of the studied orthogram. The need for such a system is justified both by the description of the diachronic analysis process and by a specific example of such an analysis. Special attention is paid to the disclosure of such important concepts for the theory of writing as "usage", "norm", "codification" and the relationship between these concepts. Special attention is paid to the question of the legality of the use of the term "customary norm", for which the arguments confirming the incorrectness of this term are considered in detail. In addition, the problems of correlation between the principles of spelling as tools of descriptive (descriptive) spelling and the rules as a result of the activities of orthologists codifying the norms of writing are discussed. The system of criteria for descriptive spelling is based on the system of rules of prescriptive spelling, namely the rules of modern spelling from the new electronic resource "Spelling commentary of the Russian Dictionary" (OROSS) by E. V. Beshenkova, O. E. Ivanova, E. V. Tenkova. The novelty of the results obtained in the article is due to the lack of research methodology in the field of descriptive diachronic spelling. The paper reveals such important concepts for the theory of writing as "usage", "norm", "codification" and clarifies the relationship between these concepts. As a result, the illegality of the use of the term "customary norm" was established, for which the arguments confirming the incorrectness of this term were considered in detail, in particular, that in certain contexts the terms "usage" and "norm" can be used indifferently. The result of the work was a system of criteria for choosing spelling options at different stages of the formation of the orthogram. The development of an original system of criteria for choosing spelling variants was carried out on the basis of a modern set of spelling rules. The scope of the research results is determined by describing the process of diachronic analysis of the orthogram and illustrated by a specific example of such analysis.