The methyl cyanide J = 12_K−11_K (top), 13_K−12_K (middle), and 14_K−13_K (bottom) spectra observed towards IRc2, identifying the various lines detected in these frequency ranges. The K-ladder lines of the main isotopologue of methyl cyanide are labelled in red, those of the CHCN isotopologue are labelled in light blue, and emission lines from other species are indicated in grey. Note that the J = 12₉−11₉ transition suffers strong contamination from the ¹³CO J = 2−1 line.