Petersen diagram for multimode classical Cepheids. Different types of multi-periodicity are plotted with different colors and symbols. Blue circles: Pulsations in fundamental (F) mode and first overtone (1O). Red squares: Pulsations in 1O and second overtone (2O). Cyan asterisks: Pulsation in 2O and third overtone (3O). Green triangles: Pulsations in 1O and 3O. Lime diamonds: Pulsations in F and 2O. Orange points: 0.61 Cepheids. Grey pluses: Subharmonics in 0.61 Cepheids. Data are combined from different sources (see references in Netzel (2023)). Signals detected in X Sgr are plotted with black asterisks and are labeled.