Some important correlations between medium-term economic cycles (7–11 years) known as Juglar cycles and long (40–60 years) Kondratieff cycles are presented in this paper. The research into the history of this issue shows that this aspect is insufficiently studied. Meanwhile, in our opinion, it can significantly clarify both the reasons of alternation of upswing and downswing phases in K-waves and the reasons of relative stability of the length of these waves. It also can provide the certain means for forecasting. The authors show that adjacent 2–4 medium cycles form the system the important characteristic of which is the dynamics of economic trend. The latter can be upswing (active) or downswing (depressive). The mechanisms of formation of ...