Loomis-Wood-type plot obtained from the AABS programs package (Kisiel et al. 2005) showing the identification of the stripes corresponding to the sequences of the b-type R-branch transitions for , and 4. Each sequence has two components, A and E, due to the methyl internal rotation. Rotational transitions are lined up to the central frequencies ν_cent of the A-component transitions with the rotational quantum numbers indicated on the right side of the diagram. The frequency distance Δν from the central frequency is documented on the x-axis. Each A-E doublet going to the left from the leading one at Δν = 0 corresponds to increasing K_a and decreasing J transitions. The highlighted horizontal stripe represents the part of the spectrum documented in Fig. 2b. For J′′< 60, the degeneracies for the energy levels with K_a = 3 and K_a = 4 are lifted and two separate sequences of (the left one) and (the right one) transitions are recognizable for both A- and E-components.