FTMW spectrum of the anti-conformer of 3-methylbutyronitrile is shown. The spectral lines are split in the jet-experiment into two Doppler components since the microwave emission is parallel to the supersonic jet expansion. The actual frequency is indicated and is determined by calculating the arithmetic mean of the two Doppler components. The hyperfine splitting due to the ¹⁴ N nucleus is obvious. Here, the transition J_Ka,Kc = 3_1,3 –2_0,2 is split into its five components. The superscript gives the F quantum numbers of the respective transition.