The existence of bosons as preformed pairs in chemical equilibrium with their decay products, the pairing fermions in the normal states of high-T c superconductors, is used to show that one obtains in general a parabolic formula for the transition temperature T c (x). This is applied to the case of La 2 - x M x CuO 4 (M = Sr, Ba) to explain their T c -formulas which deviate from the parabolic behavior. The main idea is that the orthorombic and tetragonal crystal lattices have different onset values x 1 and maximum overdoping values x 2 . For Ba-compunds the regions of superconductivity do not overlap for the two structure types.