Geometrical relation of transitions and critical points in phase diagrams of the one com-ponent system was discussed thermodynamically. Only transition of odd order of the Ehrenfest classification is possible along a finite length of phase boundary. The critical points was defined as an intersecting point of a first order transition line and a stability limit of the state. So-called second order transition was critisized. The discussion was extended to three dimen-sional space, and some two dimensional phase diagrams were understood as the subspace. Ferro-electrics and antiferroelectrics were taken as pedagogical examples of three dimensional phase diagram and tricritical point.
