Laboratory frequencies (or predicted frequencies from laboratory data) minus observed frequencies in IRC +10216 (in kHz) as a function of the frequency of the lines. The different symbols correspond to the following molecules: SiC₂ (red filled squares); ²⁹SiC₂ and ³⁰SiC₂ (red empty squares); HC₃N (blue filled squares); the three ¹³C isotopologues of HC₃N (blue empty squares); HC₅N (cyan filled squares); C₄H (cyan empty squares); ¹³CS, C³⁴S, C³³S, ¹³C³⁴S, CCS, and CCCS (green filled squares); ²⁹SiS, ³⁰SiS, Si³⁴S, Si³³S, and ²⁹Si³⁴S (black empty squares).