  The origin of interband electron pairing responsible for enhancing
superconductivity and the factors controlling its strength were examined. We
show that the interband electron pairing is a natural consequence of breaking
down the Born-Oppenheimer approximation during the electron-phonon
interactions. Its strength is determined by the pair-state excitations around
the Fermi surfaces that take place to form a superconducting state. Fermi
surfaces favorable for the pairing were found and its implications were
discussed.
