Petersen diagram showing the location and stability information for the computed non-linear convective models. The models were chosen to cover the significant part of the Petersen diagram. These are models of 2.5M_⊙, 3.0M_⊙ and 3.5M_⊙ located along evolutionary tracks (Δlog L = 0.0) and models with artificially increased luminosity of masses 3.5M_⊙ (Δlog L = 0.2), and 2.5M_⊙ and 3.0M_⊙ (Δlog L = 0.4). All computations are for the metallicity parameter Z = 0.006. Black dots represent the observed LMC double-overtone Cepheids. The only double-periodic models are found near the 2ω₁ = ω₅ resonance centre, which is marked with an arrow.