Interstellar gas offers advantages when quantifying the structure of the resonances in disc galaxies. The gas is concentrated close to the disc mid-plane, making it easier to determine its in-plane velocity components, and it offers sharp emission lines for good velocity resolution. These properties are shared by atomic, molecular, and ionized gas; high spectral and angular resolution over wide fields can be performed, especially for the ionized and molecular phases. We have previously shown that using gas kinematics corotation radii can be found, not only corresponding to the main bars of barred galaxies, but also to nuclear bars, and for the spiral arms, which usually exhibit several corotations, corresponding to a concentric annular density wave pattern. We have also shown that a gas kinematic technique gives precise correspondence with the structure found using the Tremaine Weinberg method for stars, but can cover a larger fraction of galaxy discs. In this contribution we present our recent analysis of the resonant structure of M51, using kinematic mapping with the CO molecule from original PAWS data supplemented with our own velocity map in H We demonstrate the role of its interacting satellite NGC 5195 in stimulating the formation of the spiral arms, and demonstrate that it has twice passed through the plane of M51, calculating the time interval between passages. Our results are strengthened by comparison with simulations of this interaction.