Current paper presents results of recent studies of the mechanisms of constriction and stratification of the positive column of a glow discharge in neon and argon. The causes of plasma compression into a thin cord are discussed, among which one can point out the inhomogeneous heating of the neutral gas and the kinetic effect associated with the depletion of the distribution function of fast electrons along the discharge tube radius. Basing on the experiments, it was shown that under considered discharge conditions (pressure of 10 - 100 Torr, currents of 10 - 100A) the inhomogeneous gas heating is not the basic mechanism of constriction. The analysis of the stationary solution for a positive column on the stability with respect to two-dimensional wave disturbances was carried out. It is shown that the diffuse discharge is stable and essentially two-dimensional striations are propagating in the constricted cord.