Context. Radio bursts with fine structures are used in diagnostics of solar flare plasmas, of which zebra structures are the most important. However, there is still a debate about their origin. Aims. The most probable model of zebras is that based on double-plasma resonance (DPR) instability. The paper wants to contribute to a verification of this model. Methods. We used analytical methods. Results. We studied the DPR model in two scenarios: A model with the zebra-stripe sources in a single loop and a model with the zebra-stripe sources moving through a fan of magnetic field lines. In the first case, we found several new relations among the parameters of zebra stripes and their sources, which can be used to analyze observed zebras and thus to verify if the zebra is generated according to the DPR model. These relations were derived for the zebra-stripe sources distributed along the loop and also for those having some extent in the loop radius. In the scenario with the moving zebra-stripe sources, we determined the parameters of the 14 December 2006 zebra and estimated a change of the ratio of magnetic field and density scales causing the change of zebra-stripe frequencies. In this case we found that this zebra can be also explained in the model with the zebra-stripe sources in a single loop. Both the interpretations are discussed.