Linear relations between the FWHM of Hα and the FWHM of Hβ, Paα, Paβ, or He i, from left to right and top to bottom. Red filled circles denote simultaneous observations of the two lines, while black filled circles describe non-simultaneous line measurements. Grey open squares indicate the measurements that are classified as outliers (see text for more details) and are not considered in the fits. The black dotted line shows the 1:1 relation in all panels. In the top left panel, the best-fit relation computed on the coeval sample is shown as a red solid line. The relations from Greene & Ho (2005, dashed cyan) and Mejía-Restrepo et al. (2016, dashed blue) are also reported.