Upper panel: the water maser spectrum of NGC 4051 produced by the combination the two separate IFs used in the VLA A-array observation. The letters indicate the main features constituting the line complex. The (a), (b), and (c) labels in the spectrum indicate the three groups of features discussed in Sect. 4.2. Lower panel: VLA spectra of the maser emission in NGC 5506, taken with the VLA DnC (solid line) and A (dashed line) arrays, respectively. Given the narrow band used in the VLA A-array measurements, no line-free channels were present. Hence, the spectrum shown is most likely the portion of maser emission in that velocity range plus a small amount of continuum emission. The zero-flux density line has been added to help visualize this aspect.