Observed lines of CH₃CH₂CCH in TMC-1. The line parameters are given in Table A.1. The abscissa corresponds to the rest frequency assuming a velocity for the source of 5.83 km s⁻¹. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. Quantum numbers are indicated in the top right corner of each panel. The red line corresponds to the synthetic spectrum derived from the LTE model described in Sect. 3.1. Blanked channels correspond to negative features produced in the folding of the frequency-switching data. For some lines, we found a frequency shift of up to ±40 kHz with respect to the predictions (see Sect. 3.1). This is indicated in magenta in the corresponding panels.