H I spectra of PNe. For each subfigure, the top and bottom panels show the ON−OFF and OFF−OFF spectra, respectively. The arrows mark the systemic velocity position the PNe. The vertical dotted line indicates the velocities of the blue end of optical emission lines ([N II] or Hα lines), with the exception of IC 4846, where no such emission lines are found in the literature. A fitting to the potential circumstellar H I absorption is shown by the dashed curves.