Spectral index between 150 MHz and 1.4 GHz vs. linear size of COMP2CAT sources. The red lines mark the criteria chosen by Labiano (2006) to define CSSs: projected linear sizes below 15 kpc and α_{0.15 − 1.4} ≳ 0.5 (corrected using the cosmology adopted). The area shaded in red corresponds to sources that would be considered CSSs according to these criteria. The black lines correspond to the criteria presented by Orienti & Dallacasa (2014): projected linear sizes smaller than 20 kpc and α_{0.15 − 1.4} ≳ 0.7 (also corrected using our cosmology). The area shaded in gray marks the CSS area in the diagram based on the Orienti & Dallacasa (2014) selection.