  Over the recent years, the Petersen diagram for classical pulsators, Cepheids
and RR Lyr stars, populated with a few hundreds of new multiperiodic variables.
We review our analyses of the OGLE data, which resulted in the significant
extension of the known, and in the discovery of a few new and distinct forms of
multiperiodic pulsation. The showcase includes not only radial mode pulsators,
but also radial-non-radial pulsators and stars with significant modulation
observed on top of the beat pulsation. First theoretical models explaining the
new forms of stellar variability are briefly discussed.
