Nuclear spectra of the 33 3C sources with a detection of the [O III] emission line. Superposed on the [O III] profile (blue) is the mirror image of the redshifted line (black). Their difference (red) reveals any line asymmetry. We estimated the median velocity (V₅₀) of the asymmetric line wing and its flux (F_w), after masking the spectral region at one-third of the maximum height of the profile (grey region).