¹²CO J = 10–9 (left) and ¹³CO J = 10–9 (right) spectra of low-mass class 0 (LM0), low-mass Class I (LMI), intermediate-mass (IM) and high-mass YSO (HM) averaged independently and compared. All spectra were shifted to 0 km s^-1, rebinned to 0.27 km s^-1 and the intensity of the emission line scaled to a common distance of 1 kpc before averaging. The green line indicates the continuum level and the red dashed line the 0 km s^-1 value. W43-MM1 was not included in the average of ¹³CO J = 10–9 high-mass spectra because of the strong absorption features caused by H₂O⁺ (see Sect. 2.2).