  We show that fully aligned neutron-proton pairs play a crucial role in the
low-energy spectroscopy of nuclei. with valence nucleons in a high-j orbital.
Their dominance is valid in nuclei with valence neutrons and protons in
different high-j orbitals as well as in N = Z nuclei, where all nucleons occupy
the same orbital. We demonstrate analytically this generic feature of the
neutron-proton interaction for a variety of systems with four valence nucleons
interacting through realistic, effective forces. The dominance of fully aligned
neutron-proton pairs results from the combined effect of (i) angular momentum
coupling and (ii) basic properties of the neutron-proton interaction.
