Observed transitions of C₄S observed towards TMC-1. The abscissa corresponds to the local standard of rest velocity of the lines adopting the rest frequencies given in Table B.1. The ordinate corresponds to the antenna temperature corrected for atmospheric and telescope losses in mK. The derived line parameters of the observed lines are given in Table B.1. The synthetic spectra (red line) are derived for N=3.8x10¹⁰ cm⁻² and the LVG analysis described in section A.7.