  A notable aspect of high-temperature superconductivity in the copper oxides
is the unconventional nature of the underlying paired-electron states. The
appearance of a resonance peak, observed in inelastic neutron spectroscopy in
the superconducting state of the High Tc cuprates, its apparent linear
correlation with the critical superconducting temperature of each of the
compounds and its disappearance in the normal state are rather intriguing. It
may well be that this peak is the signature of the singlet to triplet
excitation, and is an unique characteristic of a d-wave superconductor. We
develop a simple criterion for the resonance peak which is based on the concept
of twist sti ness and its disappearance at T=Tc.
