It is shown that the linear systems, with the relative degree of the open loop dynamics equal to one and the plants with minimum phase zeros, are stable for very high gain (k → ∞) of the proportional controller. It is also shown that entering the saturation nonlinearity to these systems, which cuts the high peaks of the control, does not change their essential properties. Basing on that, the Criterion of the Relative Degree One (RD1) is formulated. In the examples containing the results of simulations, the Criterion RD1 is used for designing the systems with the plants: linear, nonlinear and with time varying parameters. It is noted that the designed systems are insensitive to the large and fast parameter changes.