  It is demonstrated that the transition temperature (Tc) of high-Tc
superconductors is determined by their layered crystal structure, bond lengths,
valency properties of the ions, and Coulomb coupling between electronic bands
in adjacent, spatially separated layers. Analysis of 31 high-Tc materials
(cuprates, ruthenates, rutheno-cuprates, iron pnictides, organics) yields the
universal relationship for optimal compounds, kBTc0 = {\beta}/\ell{\zeta},
where \ell is related to the mean spacing between interacting charges in the
layers, {\zeta} is the distance between interacting electronic layers, {\beta}
is a universal constant and Tc0 is the optimal transition temperature
(determined to within an uncertainty of +/- 1.4 K by this relationship).
Non-optimum compounds, in which sample degradation is evident, e.g. by
broadened superconducting transitions and diminished Meissner fractions,
typically exhibit reduced Tc < Tc0. It is shown that Tc0 may be obtained from
an average of Coulomb interaction forces between the two layers.
