Line blending effects as a function of the velocity difference δV_lsr between two adjacent components separated three (left), two (center), and one (right) times their intrinsic linewidth ΔV_int. Central line opacities of τ₀ = 0.5 (blue), τ₀ = 5 (red), and τ₀ = 100 (black) are considered as representative of the three main CO isotopologues, C¹⁸O, ¹³CO, and ¹²CO, respectively. The central velocities of the two components forming each spectrum are indicated with blue lines. In the case of complex spectra with multiple components close in velocity (δV_lsr ≲ 2ΔV), the individual lines can only be identified using optically thin tracers.