The methyl cyanide J = 6_K−5_K spectrum observed towards IRc2 (the central position of the map). The K-ladder lines of the main isotopologue of methyl cyanide are labelled in red and those of CHCN are labelled in light blue. Note that the K = 5 transition of methyl cyanide is contaminated by the CHCN J = 6₀−5₀ and J = 6₁−5₁ lines, as is the case for all K-ladder series.