Sample Loomis-Wood type plots of the rotational spectrum of CH₃NCO for several assigned line sequences for ν_b = 0 where sections of the experimental spectrum are plotted around the indicated centre frequencies from the linear-type fits. The plots illustrate how the assignments reached with Stark spectroscopy at J′′ ≤ 3 could be unambiguously transferred to the millimetre wave region. The blue lines mark the estimated frequency uncertainty.