  We compare the Tcs found in different families of optimally-doped High-Tc
cuprates and find, contrary generally accepted lore, that pairing is not
exclusively in the CuO2 layers. Evidence for additional pairing interactions,
that take place outside the CuO2 layers, is found in two different classes of
cuprates, namely the charge reservoir and the chain layer cuprates. The
additional pairing in these layers suppresses fluctuations and hence enhances
Tc. Tcs higher than 100K, are found in the cuprates containing charge reservoir
layers with cations of Tl, Bi, or Hg that are known to be negative-U ions.
Comparisons with other cuprates that have the same sequence of optimally doped
CuO2 layers, but have lower Tcs, show that Tc is increased by factors of two or
more upon insertion of the charge reservoir layer(s). The Tl ion has been shown
to be an electronic pairing center in the model system (Pb,Tl)Te and data in
the literature that suggest it behaves similarly in the cuprates. A number of
other puzzling results that are found in the Hg, Tl, and Bi cuprates can be
understood in terms of negative-U ion pairing centers in the charge reservoir
layers. There is also evidence for additional pairing in the chain layer
cuprates. Superconductivity that originates in the double "zigzag" Cu chains
layers that has been recently demonstrated in NMR studies of Pr-247 leads to
the suggestion of a linear, charge 1, diamagnetic quasiparticle formed from a
charge-transfer exciton and a hole. Other properties of the chain layer
cuprates that are difficult to explain using models in which the pairing is
solely confined to the CuO2 layers can be understood if supplementary pairing
in the chain layers is included. Finally, we speculate that these same linear
quasi-particles can exist in the 2-dimensional CuO2 layers as well.
