The article examines the main stages in the formation and development of the theory of the period: there is an analysis of the interpretation of this form in the works of A.B. Marx, G. Riemann, the Russian musicologists I. Sposodin, Yu. Tyulin, L. Mazel, V. Tsukerman and others. The dual nature of the period is characterized in connection with its simultaneous belonging to the syntactic and compositional levels of musical form. There are two approaches to understanding this structure – metro-tectonic and content-functional. It is concluded that the qualities of expositionality and completeness of the period are provided by its attraction to the periodicity of the structure, harmonic closure and functional balance of the constituent elements