Observed transitions in TMC-1 of the ²Π_3/2 (two left columns) and ²Π_1/2 (two right columns) states of C₆H. The labels e and f correspond to the Lambda-type doubling components of each transition. The abscissa corresponds to the LSR velocity. The derived line parameters are given in Table B.1. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in mK. Blanked channels correspond to negative features produced when folding the frequency-switched data. The quantum numbers of each transition are indicated in the corresponding panel. The red line shows the computed synthetic spectra for this species (see Sect. 3).