[en] Collective nuclear states are described by symmetric couplings of proton and neutron pairs. These J=0 and J=2 pairs are respresented by s- and d-bosons respectively. The multiple structure of the combined system is given by representations of the SU(6)xSU(2) group, the Arima-Iachello interacting bosons corresponding to the fully symmetric ones. The validity of the boson picture is attributed to the attractive proton-neutron interaction which is also responsible for the transition from vibrational to rotational spectra. (author)