The fundamentals of the blade-screw theory have been lately the subject of numerous discussions. Most of these discussions are the result of misunderstandings arising from the fact that the fundamental conceptions and quantities involved in the subject are left without sufficient definitions. The blade-screw problem is very complex. Its study requires the use of many new conceptions and physical laws in generalised form. For the last reason, special attention must be paid to their exact statements. The following remarks are intended to bring attention to some of the fundamentals of the subject which seem to be the main source of the misunderstandings which are the most widespread at the present time. All the considerations that follow are quite general; they take place for any blade-screw, but it is the propulsive blade-screw or propeller that we will chiefly have in view here.