On the basis of X-ray and tensimetric equilibrium experiments and well-known literature data a detailed analysis of (123) polymorphism has been done. The phase relationships involving the next (123) polymorhs : Ortho-I, Ortho-II, Ortho-III, Tetr and two modifications of high pressure : (124) and (247) are represented as a (T, x) section at ambient oxygen pressure and a (T, x, y) phase diagram in the presence of melt.