The paper uses the method of dividing mechanical gears into simple mechanisms (“primary”). A definition and a complete nomenclature of the simplest mechanisms are introduced. Mechanisms based on the interaction of bodies along an inclined plane are investigated. Mechanical systems are considered as conservative, holonomic, and closed. Their functional characteristics are critically evaluated and compared. The domain of existence is defined as a function of the degrees of freedom of the links. The range and boundary values of the geometric characteristics of these mechanisms, functional dependencies of the input parameters are highlighted. A criterion for separating the simplest mechanisms is introduced: “differential wedge pair” and “inclined plane”. The analysis of approaches to the consideration of the concept of self-braking and criteria for its reliability is carried out. The methods of studying the simplest gears through the “coefficient of friction” and “angle of friction” are compared. A characteristic and a method for quantifying the “reliability of self-braking” are proposed.