Top left panel: measured major axis (FWHM) in arcsec vs. the FIRST flux density for sources that do not show clearly resolved radio emission; the red dashed line marks the adopted limit of 6.̋7 for the inclusion in the FR0CAT sample. Top right panel: distribution of the NVSS fluxes (left) of the 108 FR0CAT (blue), the 219 FRICAT (black), and the 14 sFRICAT sources (red). The vertical dotted line marks the 5 mJy limits of the BH12 sample. Bottom left panel: ratio between the FIRST and NVSS fluxes vs. the NVSS flux density for the FR0CAT sources; the dashed curve indicates the upper boundary of the region in which objects are located that pass the NVSS flux threshold, but were not selected due to the 5 mJy minimum FIRST flux requirement because of a large contribution of resolved emission. Bottom right panel: radio spectral index between 327 and 1400 MHz; the dashed cyan histogram represents the upper limits.