Authors investigate the phenomenon of customary law and the practice of applying customs by Russian courts. The theory of legal custom developed in continental legal doctrine from Roman law to the era of European codifications. This theory developed the main features of legal custom and ways of proving its existence, which later became the basis of modern private law in continental Europe. The legal doctrine formulates a number of requirements that legal customs must satisfy - requirements for the actions from which the custom is formed, requirements for subjective meaning given to these actions by the actors themselves and third parties, as well as requirements for the content of customs. Comparing the legal concept of custom to the socio-legal concept proposed by M. Weber, the authors note their similarities in essential features. Examining historical dynamics of the relationship between customary law and codified law, the authors have identified trends characteristic for both the Romano-Germanic (Сontinental) legal family as a whole and the Russian legal culture that is part of it. The paper offers an explanation of these trends. Having analyzed the customary-applying practice of Russian courts, the authors propose its sociological and legal interpretation, in particular, they note the confirmation of the hypothesis about the particularization of customary law and rudimentary existence of traditional customary law in modern culture.