The use of Petersen diagrams and period ratios in investigating the pulsational content of stars in the classical Instability Strip

The aspects of the analysis of photometric time-series obtained on double-mode or multiperiodic pulsating stars are briefly reviewed. In particular, the ratios between frequencies are used to pin cases revealing peculiarities. In addition to the Petersen diagrams, we also demonstrated that the period ratios can detect interesting objects. In particular, new results are obtained on High–Amplitude Delta Scuti contained in the OGLE-II database.