Panel a: moment-1 HI map in colour with the moment-0 contours from Fig. 1b in black. The CLEAN beam is displayed as a black ellipse in the top left corner. Panel b: position-velocity diagram extracted along the dashed line in panel a; an 11.3′ slice from RA Dec −00°57′19.85′′ to RA Dec −00°50′48.73′′, that is, passing through both NGC 4418 and VV 655. Four distinct features can be seen along the slice at the approximate relative offset values of −200′′, −40′′ (VV 655), 152′′ (NGC 4418; in absorption), and 212′′. Spectra extracted at these positions, that is, the dashed lines in panel b, are presented in Fig. 3. Contours in panel b are ±[0.025,0.05,0.1,0.2,0.4,0.8] × 26.7 mJy/beam (the peak).