Some of the lines used in the fit to obtain the rotational constants of SiC₂ and its isotopologues. The fitted line profile is shown in red. Line identification is provided in each panel. Abscissa is frequency in MHz and the intensity scale corresponds to T_A^*. Numbers in each panel indicate the difference between laboratory frequencies (or predicted frequencies from laboratory data) minus the derived frequency of the lines observed towards IRC +10216 using the SHELL fitting method (see text).